Adding and subtracting decimals
Same method as whole numbers, place by place.
The cloth
Priya needs 2.7 m of cloth for her skirt. Shylaja needs 3.5 m for her kurti.
What is the total cloth needed?
What this lesson covers
The idea
The standard procedure for adding and subtracting whole numbers can be used to add and subtract decimals, working place by place: tenths with tenths, hundredths with hundredths, and so on.
The cloth
Priya needs 2.7 m of cloth for her skirt. Shylaja needs 3.5 m for her kurti.
What is the total cloth needed?
- 5.12 m
- 6.2 m
- 10.2 m
Add and subtract place by place
Work tenths with tenths, then units. When a column has 10 or more, tap 10 → 1. If a column is short in a subtraction, tap 1 → 10.
The same method
Line up the decimal points. Then tenths meet tenths, hundredths meet hundredths, exactly as ones meet ones in whole numbers. Here is 75.345 + 86.691:
The standard procedure for adding and subtracting whole numbers works for decimals: work place by place, tenths with tenths, hundredths with hundredths, and so on.
Notes
Add and subtract decimals like whole numbers, place by place: tenths with tenths, hundredths with hundredths. Regroup 10 of a place into 1 of the next place.
Check yourself
What is 2.7 + 3.5?
Answer: 6.2 m
2.7 + 3.5 = 6.2. The total cloth needed is 6.2 m.
What is 3.5 − 2.7?
Answer: 0.8 m
3.5 → 2 units 15 tenths. 15 − 7 = 8 tenths and 2 − 2 = 0 units: 0.8.
What is 1.25 + 2.8?
Answer: 4.05
1.25 + 2.80: hundredths 5, tenths 2 + 8 = 10 → 0 tenths and 1 unit, units 1 + 2 + 1 = 4. So 4.05.
Shylaja’s hand is 12.4 cm long and her palm is 6.7 cm. How long is her longest finger?
Answer: 5.7 cm
12.4 → 11 units 14 tenths. 14 − 7 = 7 tenths and 11 − 6 = 5 units: 5.7.