Decimals · Usually met around P4
Adding tenths and hundredths
Turn tenths into hundredths, then add.
How to do it
Method
Turn tenths into hundredths, then add
310 + 4100 = 30100 + 4100 = 34100
Another way
Write decimals
0.3 + 0.04 = 0.34
In full
A row of a hundred square is 10 squares: one row is 110 of it, and also 10100. So 110 = 10100 and 310 = 30100: the top and bottom both times 10. You can only add the tops when the bottoms match, so first turn the tenths into hundredths: 310 + 4100 = 30100 + 4100 = 34100. On a hundred square that is 3 rows and 4 more squares, 34 in all. Never add the tops and the bottoms separately (310 + 4100 is not 7110). Another way: write them as decimals and add, 0.3 + 0.04 = 0.34 = 34100, the same answer.
Worked examples
Have a go first, then open each one for the answer and the working.
Example 1
810 = □/100
Show the answer and working
Answer: 80
One row of the hundred square is 110 and also 10 squares: 8 rows are 80 squares, so 810 = 80100.
Example 2
410 + 57100 = □/100
Show the answer and working
Answer: 97
410 = 40100, and 40 + 57 = 97: 97100.
Example 3
What is 210 + 6100?
- A. 8100
- B. 8110
- C. 26100
- D. 2610
Choose the right answer
Show the answer and working
Answer: C. 26100
210 = 20100; add 6100: 26100.
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