Unknown fraction

Algebra Level 2

Given the fraction

A B , A < B \frac { A }{ B } ,\quad A<B

The difference between A A and B B is 2 2 . The value of the fraction increases 1 / 6 1/6 when adding 2 2 to A A and 3 3 to B B . What is the fraction?

3/5 1/3 1/2 5/7 7/9

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1 solution

Stephen Mellor
Jan 17, 2018

Since A < B A < B and the difference between A A and B B is 2 2 , we can write B = A + 2 B = A + 2 . This turns the original fraction into A A + 2 \frac{A}{A+2} . We can write the information in the question as A A + 2 + 1 6 = ( A ) + 2 ( A + 2 ) + 3 \frac{A}{A+2} + \frac{1}{6} = \frac{(A) + 2}{(A+2) + 3} A A + 2 + 1 6 = A + 2 A + 5 \frac{A}{A+2} + \frac{1}{6} = \frac{A + 2}{A+5} Multiplying through by 6 ( A + 2 ) ( A + 5 ) 6(A+2)(A+5) , 6 A 2 + 30 A + A 2 + 7 A + 10 = 6 A 2 + 24 A + 24 6A^2 + 30A + A^2 + 7A + 10 = 6A^2 + 24A + 24 A 2 + 13 A 14 = 0 A^2 + 13A - 14 = 0 Hence, A = 14 A = -14 or A = 1 A = 1 . The only sensible solution is then 1 3 \boxed{\frac{1}{3}}

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