The sum of the ages

Algebra Level 2

The ratio of the sum of the ages of A , B A,B and C C twenty years from now to the sum of their ages eight years ago is 23 : 9 23:9 . If B B is two years older than A A and C C is twice as old as B B , find the present age of C C .


The answer is 40.

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1 solution

23 9 = A + B + C + 60 A + B + C 24 \dfrac{23}{9}=\dfrac{A+B+C+60}{A+B+C-24} ( 1 ) \color{#D61F06}(1)

B = 2 + A B=2+A ( 2 ) \color{#D61F06}(2)

C = 2 B C=2B ( 3 ) \color{#D61F06}(3)

Simplifying ( 1 ) \color{#D61F06}(1) , we get

23 9 = A + B + C + 60 A + B + C 24 \dfrac{23}{9}=\dfrac{A+B+C+60}{A+B+C-24}

23 A + 23 B + 23 C 552 = 9 A + 9 B + 9 C + 540 23A+23B+23C-552=9A+9B+9C+540

14 A + 14 B + 14 C = 1092 14A+14B+14C=1092

Substitute ( 2 ) \color{#D61F06}(2) and ( 3 ) \color{#D61F06}(3) into the above equation

14 A + 14 ( 2 + A ) + 14 ( 2 B ) = 1092 14A+14(2+A)+14(2B)=1092

14 A + 14 ( 2 + A ) + 14 ( 2 ) ( 2 + A ) = 1092 14A+14(2+A)+14(2)(2+A)=1092

14 A + 28 + 14 A + 56 + 28 A = 1092 14A+28+14A+56+28A=1092

56 A = 1008 56A=1008

A = 18 A=18

It follows that B = 20 B=20 and C = 40 C=40

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