SOLUTION: John is 4 years old & his sister is three times as old as he. When John is 12, how old is his sister ?

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So when John is 4 years old his sister is three times as old as he, which means that she is 4*3 = 12 years old at that time, so as John become 12, meaning that 8 years passed, so add 8 to his sister's age, 12+8 = 20, so his sister is 20 years old when John is 12 years old.