42 is 3 times 14.
Joan's daughter is 14 years old.
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If you know your times tables, you know 42 = 6·7. Since you're familiar with the associative property of multiplication, you know you can regroup the factors of this:
... 42 = 6·7 = (3·2)·7 = 3·(2·7) = 3·14
Of course, you can always simply divide 42 by 3 using your calculator or even long division.