7. A is four times as old as B. Ten years ago, A was 10 times as old as B. Find the present age of B.
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Let present age of B be X year's.
Present age of A = 4 × Present age of B = 4X year's.
Before 10 year's age of A = Present age of A - 10 = ( 4X - 10) year's.
Before 10 year's age of B = Present age of B - 10 = ( X - 10 ) year's .
According to question,
Before 10 year's age of A = 10 ( Before 10 year's age of B ).
4X - 10 = 10 ( X - 10)
4X - 10 = 10X - 100
10X - 4X = -10 + 100
6X = 90
X = 90/6
X = 15 year's.
Present age of B = X = 15 year's
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