Line up the points, not the ends
Two ribbons are 2.7 m and 1.45 m long. Joined, how long are they?
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Step 1 of 4
- 2.70 and 1.45, points lined up.
- Hundredths: 0 + 5 = 5.
- Tenths: 7 + 4 = 11. Carry 1.
- Ones: 2 + 1 + 1 = 4. Answer: 4.15.
The rule
Together they are 4.15 m long.
Only matching columns add. Tenths add to tenths. Hundredths add to hundredths. Parts of different sizes cannot be added as they are.
So line up the points, not the right ends. Write a zero to fill an empty column.
Subtracting works the same way. To subtract 1.75 from 5, write 5 as 5.00. Then 5.00 − 1.75 = 3.25.
Watch out!
3.7 + 0.5 is not 3.12. Seven tenths and five tenths make twelve tenths. Twelve tenths is 1 whole and 2 tenths. So the answer is 3 + 1 = 4 wholes and 2 tenths: 4.2.