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CCSS.MATH.CONTENT.3.MD.A.1

## Calculating Time – Adding and Subtracting Time

Imagine this: you want to go on a thrilling scavenger hunt but to succeed, you must follow the clues and get to each one in time! For this, you'll need the superpower of adding and subtracting time. Read on, and soon, you'll be a time calculation wizard!

Wondering how to play with hours and minutes? There are many magical methods for addition and subtraction with time, but one of the best tools is a number line.

## Adding Time Using a Number Line

When adding time with a number line, always start on the left with your starting time. From there, skip count forward by the amount you're adding.

You can leap forward in fives, tens, or even more. The spot you end up on shows the new time. And guess what? This method works when adding both hours and minutes!

For instance, imagine you start your hunt at 2:20 and spend 35 minutes searching. To find out when you'll uncover the next hint, use an open number line. Starting at 2:20 and adding 35 minutes, you'll discover the next hint at 2:55!

## Subtracting Time Using a Number Line

Subtracting is like walking backward in time! For this, always start on the right with your starting time. Then, skip count backward by the amount you're subtracting.

Hop back in fives, tens, or more to find your new time.

Imagine you have a clue that says you need to reach a location by 5:30. But, the journey will take 25 minutes. When should you start? Use the open number line to find out. If you need to be somewhere by 5:30 and the journey takes 25 minutes, start your trip by 5:05 to be right on time!

## Adding and Subtracting Time – Summary

Whether you are adding or subtracting time, remember to use a number line and follow the steps to success.

Step # What to do
1 Begin with your starting time on the left side of the number line.
2 Hop forward in increments of the time you're adding.
3 The spot you land on is your new time!

When subtracting time:

Step # What to do
2 Skip backward by the time you're subtracting.

With these tools in hand, you're ready to ace any time challenge and become a master of the clock!

## Frequently Asked Questions on Calculating Time

How do I add time when one of them is in PM and the other in AM?
Why do we sometimes subtract time to find out the duration of an event?
Why is it essential to understand the 12-hour and 24-hour formats?
Why do minutes and hours have a base of 60 instead of 100 like regular arithmetic?

"What's this, a letter? Oh, it's from Zuri!" : "I need to follow the clues to find a surprise!" : "Hmm (...) the next clue will be at the fountain." Freddie needs to be at the fountain to find the next clue, but what time will he get there? "Adding and Subtracting Time" It will take Freddie fourty-five minutes to get to the fountain. If he leaves at 1:15, what time will he get there? We can calculate this by adding time on a number line. When adding using a number line you always put the starting number on the LEFT side. Let's put 1:15 on the left side. Next we can hop forward small amounts of how much time it will take. It will take Freddie fourty-five minutes (...) so we can hop forward fourty-five minutes. We can skip count by tens to help. Ten (...) twenty (...) thirty (...) forty. Then, we can count five minutes more to find out the ending time. After skip counting forty-five minutes forward, we found the ending time (...) two o'clock. Freddie will get to the fountain at two o'clock. : "Clue number two says to look behind the bush!" ... : "… A swimsuit!! Are we going swimming?" : "It says I need to be at the park by 2:30 to find the next clue." If it takes Freddie twenty minutes to get there, what time should he leave? We can help Freddie by using the number line to SUBTRACT time. When subtracting using a number line, you always put the starting number on the RIGHT side. Let's put 2:30 on the right side of the number line. Next, skip count backwards small amounts of the time it will take. Since it will take Freddie twenty minutes, we can skip count backwards twenty minutes. Let's skip count backwards by tens! Ten (...) twenty. We skip counted backwards twenty minutes and ended on 2:10. So, Freddie should leave by 2:10 to get there in time. : "It says my next clue is at home." : "I’ll need to take two busses that are each five minutes long to get there." If Freddie leaves right now at 2:30, what time will he get home after taking the two buses? We need to count up, or add, to find the answer. Since we’re adding, step one is to put the number on the LEFT side of the number line. Next, we need to skip count, or count up the amount of time it will take. If Freddie takes two busses that each take five minutes, when will he get home? Freddie will get home by 2:40. : "Here’s clue number four!" : "It says I need to get changed and find Zuri at the sunniest spot by 3:30 (...) hmm, the sunniest spot?" The sunniest spot is close to the landfill, and it will take Freddie thirty-five minutes to get there Freddie needs to find Zuri by 3:30. If it takes him thirty-five minutes to get there, what time should he leave? We need to count backward, or subtract, to find the answer. Since we’re subtracting, we'll put the start time on the RIGHT side of the number line. Next, we'll skip count, or count backwards the amount of time it will take. If Freddie takes thirty-five minutes to find Zuri, and needs to get there by 3:30, when should he leave? Freddie should leave by 2:55 in order to find Zuri in time! When ADDING with a number line, remember... Put the start time on the LEFT side. Next, skip count, or count forward on the number line. Then, skip count DOWN the number line until you reach the time. When SUBTRACTING time with a number line remember (...) Start by putting the time on the RIGHT side of the number line. Then, skip count, or count backwards small amounts on the number line until you reach the time. Remember, the number you count back to is the time. : "You did it! Welcome to the beach party, Freddie!" : "This doesn’t look a lot like a beach to me… oh well!"

## Adding and Subtracting Time exercise

Would you like to apply the knowledge you’ve learned? You can review and practice it with the tasks for the video Adding and Subtracting Time .
• ### What time will Freddie get home?

Hints

How could we break 45 minutes into smaller jumps on the number line?

If we start with 3:10 and add 10 minutes, we get to 3:20. Can you keep adding the other chunks of time on the number line to find out what time Freddie got home?

Solution

Freddie got home at 3:55 pm!

We can break 45 minutes down into smaller chunks on the number line.

Start by counting by ten.

Start at 3:10 pm then add ten minutes to get to 3:20 pm then another ten minutes to get to 3:30 pm then another ten minutes to get to 3:40 pm then a final ten minutes to get to 3:50 pm. Add a further five minutes to get to 3:55 pm.

• ### Can you figure out the missing times?

Hints

Remember if we're adding time, we put the starting time on the left side and add small chunks of time on the number line.

Remember if we're subtracting time, we put the time on the right side of the number line.

Solution

1. Put the time on the left side of the number line.
2. Count forward in chunks.
3. Find the time!

When we are subtracting:

1. Put the time on the right side of the number line.
2. Count backward in chunks.
3. Find the time!

The above image shows you how to solve the first problem.

• Freddie left home at 9:00 am, so we put 9:00 am on the left side of the number line.
• We know it takes Freddie 30 minutes to get to the post office, so we can split 30 into chunks of 10.
• Counting by tens, we stop at 9:10 am, 9:20 am then finally 9:30 am, which is the answer.
_________________________________________________________________________

For the second problem:

• Freddie leaves the post office at 9:40 am.
• He knows it will take him 35 minutes to get the park.
• Put 9:40 on the left side of the number line and count forward 35 minutes.
• We can do this by counting by ten to reach 10:10 am then another five to reach 10:15 am.

For the third problem:

• This time we know the start time and the end time, so we want to figure out how long it took.
• Put 10:15 am on the left of the number line and 10:40 am on the right.
• If we start by counting by ten we will reach 10:35 am after 20 minutes. We need to add another 5 to get to 10:40 am.
• If we then add 20 and 5 we get 25 minutes.

For the fourth problem:

• Freddie needs to be home by 12:55 pm and it will take him 45 minutes.
• Put 12:55 pm on the right of the number line.
• Count back 45 minutes.
• You will reach 12:10 pm.

• ### Can you solve the addition and subtraction problems?

Hints

Make sure you have written the times using digits and with a colon (:) between the hour and minutes.

If you are trying to find out how many minutes something takes:

• Put the start time on the left.
• Put the end time on the right.
• Count forward in chunks from the start to the end time, like in this example.
Solution

The first question asked you to start at 10:15 am and add on 40 minutes. The number line above shows you how you can do this.

Start at 10:15 then count by tens to 40. This will get you to 10:55 am.

____________________________________________________________________________

For the second question:

• Put 8:45 on the right of your number line.
• Count backward by tens to 20.
• Find your time: 8:25 am.

For the third question:

• Put 6:30 on the left of your number line and 7:15 on the right.
• Count by tens from 6:30. You will reach 7:10 after 40 minutes. Add another 5 to get to 7:15.
• Add 40 and 5 together to get 45 minutes.

For the fourth question:

• Put 4:45 on the right of your number line.
• Count backward by tens to reach 3:55 then another 5 to reach 3:50.

For the fifth question:

• Put 2:40 on the left of your number line and 3:05 on the right of your number line.
• Count by tens from 2:40 and you will get to 3:00 after 20 minutes. Add another 5 to get to 3:05.
• Add 20 and 5 to get 25 minutes.
• ### Can you check Zuri's times?

Hints

Remember when adding, put the start time on the left side of the number line.

Can you break the time into smaller chunks to help you solve?

Look carefully at the hour and the minutes. Only highlight the incorrect times.

Solution

When we are adding time we put the start time on the left. The number line shown is how to solve the first problem.

• We are adding 25 minutes so we can break this down into smaller chunks.
• We end at 9:40, so the first problem is correct.
_________________________________________________________________________

For the second problem:

• This gets us to 10:00, so the minutes (05) were incorrect for this problem.

For the third problem:

• This gets us to 11:05 so the hour (10) was incorrect on this problem.

For the fourth problem:

• Subtract 20.
• This gets us to 11:50 so the minutes (40) were incorrect on this problem.

For the fifth problem:

• This gets us to 1:15 so both the hour (2) and the minutes (10) were incorrect here.

• ### Can you order the steps for adding and subtracting time?

Hints

If we're adding time, we start like this on the number line.

If we're subtracting time, we start like this on the number line.

Solution

• Put the start time on the left of the number line.
• Count forward and find the time.

To subtract time:

• Put the time on the right of the number line.
• Count backward and find the time.
• ### Can you solve Freddie's puzzle?

Hints

Find the earliest time and start from there.

Think about ordering the times first, then you can figure out the difference between each.

For this number line, how could you figure out how many minutes there are in between the two times?

Solution

Start with 12:30pm , the next time in the sequence is 1:05pm so what is the difference between 12:30 and 1:05?

We can start by counting by tens from 12:30. Once we get to 30 we will be at 1:00. If we add another 5 on we will reach 1:05 so the difference between 12:30 and 1:05 is 35 minutes.

Following the above method the following sequence should be achieved:

1. 12:30 pm
2. + 35 minutes
3. = 1:05 pm
4. + 40 minutes
5. = 1:45 pm
6. + 25 minutes
7. = 2:10 pm
8. + 30 minutes
9. = 2:40 pm.

Zuri should meet Freddie at 2:40pm.