Question

Ina a class of 30 children,60% can swim. how many children cannot swim?

Answer 1

60% of the total (30) can swim
60% of 30 = (60/100)*30 = 0.60*30 = 18
So 18 children can swim

This means 30-18 = 12 cannot swim

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An alternative is to do it this way: 

We know 60% can swim so that means 40% cannot swim. 
Notice how 100% - 60% = 40%
Or in other words, the 60% and 40% add to 100%. This is because we have two choices: either the person can swim or cannot swim

Since 40% cannot swim, this means
40% of 30 = (40/100)*30 = 0.40*30 = 12
and we get the same result as before

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Final Answer: 12