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Use inverse operations and smaller known facts to find unknown numbers.
Free examples and worked answers. No account needed.
Practise the method.
Use inverse operations and smaller known facts to find unknown numbers.
Use inverse operations and smaller known facts to find unknown numbers.
Example 1
Find the missing number: 31+ ? =81.
Show worked answer
Answer: 50
- Addition and subtraction undo each other.
- Subtract the known part from the whole: 81−31=50.
Example 2
Use 6+4=10 to find 60+40.
Show worked answer
Answer: 100
- The same fact applies to tens.
- 6 tens +4 tens =10 tens =100.
Example 3
Find the starting number: ? −28=131.
Show worked answer
Answer: 159
- Undo subtracting 28 by adding 28.
- 131+28=159, so the starting number is 159.
Put it into practice.
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