Lesson 5.4 · Measurement
Length on a number line
A number line looks a lot like a ruler. You can use it to add and subtract lengths in word problems.
A number line is like a ruler
The marks on a number line are the same distance apart. Each space is 1 unit, just like each inch on a ruler.
To add a length, hop to the right. To subtract a length, hop to the left.
Hopping on a number line
- Start at the first length.
- Hop right to add. Hop left to subtract.
- The number where you land is the answer.
Worked example: Adding lengths
Jen has a ribbon that is 6 inches long. She ties on another piece that is 5 inches long. How long is it now?
Start at 6. Hop 5 to the right.
You land on 11. The ribbon is 11 inches long. ()
Worked example: Subtracting lengths
A board is 13 feet long. Dad cuts off 4 feet. How long is the board now?
Start at 13. Hop 4 to the left.
You land on 9. The board is 9 feet long. ()
Bigger hops
For bigger numbers, hop by tens first. Then hop by ones.
Worked example: Two hops
A snake is 25 cm long. It grows 12 cm. How long is it now?
Start at 25. Hop 10 to 35. Then hop 2 to 37.
The snake is 37 cm long. ()
Common mistake
Count the hops, not the marks. Your first hop starts at your number. It does not count as 1 until you land on the next mark.
Practice
Start at 7 and hop 3 to the right. Where do you land?
Type a number.
This hop shows a length being cut off. It starts at 14 and goes left to 8. How many units long is the hop?
Type a number.
A worm crawls 8 cm. Then it crawls 4 cm more. How many centimeters does it crawl in all?
Type a number.
A rope is 18 feet long. Sam cuts off 7 feet. How many feet long is the rope now?
Type a number.
A plant is 34 inches tall. It grows 14 inches. How tall is it now?
Type a number.
Ana's jump rope is 15 feet long. Ben's is 8 feet long. How many feet longer is Ana's rope?
Type a number.
A path is 50 meters long. Lily walks 23 meters, then 8 more meters. How many meters does she still have to walk?
Type a number.