Topic 7 of 8
Step 3: Adding and subtracting to 20
Measuring length in units
Step 3 · Adding and subtracting to 20 topics
Put things in order by length, and measure a length by laying equal units end to end with no gaps and no overlaps.
Ordering by length
To put three things in order, compare them two at a time. If the pencil is longer than the crayon, and the crayon is longer than the chalk, then the pencil is longer than the chalk.
A third thing can compare two things that cannot be moved. Use a string to measure the door, then hold the same string against the window.
Measuring end to end
To measure a length, lay copies of one unit, such as paper clips or cubes, end to end along it. The number of units is the length.
No gaps, no overlaps
Units must touch end to end. Gaps make the count too small, because the gaps are not counted. Overlaps make the count too big.
- Start the first unit exactly at one end.
- Lay each unit right against the one before.
- Stop at the other end and count the units.
- Use the same kind of unit all the way.
Example: Comparing lengths
A book is 6 cubes long. A box is 9 cubes long. How much longer is the box?
- Both were measured with the same cubes.
- 9 − 6 = 3.
Answer3 cubes longer
Try it yourself
Work each one on paper first, then open the answer to check.
A rope is longer than a ribbon, and the ribbon is longer than a string. Which is shortest?
Show answer
The string.
A spoon is 5 clips long. A fork is 6 clips long. Which is longer, and by how much?
Show answer
The fork, by 1 clip.
Why must units not overlap?
Show answer
Overlapping units cover the same part twice, so the count is too big.
A table is 12 hands long. Is it longer than a mat that is 8 hands long, measured by the same person?
Show answer
Yes, by 4 hands.
A crayon is 4 cubes long. How long are 2 crayons laid end to end?
Show answer
8 cubes.