One day a quarter of the class is absent and 21 children are present. How many children are there on

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One day a quarter of the class is absent and 21 children are present. How many children are there on the class when no one is away?

If 1/4 of the class is absent, this means that 1 - 1/4 is present.

Since 1 = 4/4, we have 4/4 - 1/4 = 3/4 of the class is present.

If the full size of the class is c, then we have
3/4c = 21

Typing 3/4c = 21 into the search engine, we get:
c = 28