If A : B = 3 : 4, B : C = 5 : 7 and C : D = 8 : 9 then A : D is equal to:

If A : B = 3 : 4, B : C = 5 : 7 and C : D = 8 : 9 then A : D is equal to
[A]10 : 21
[B]21 : 10
[C]3 : 7
[D]7 : 3

10 : 21
A : D = \frac{A}{D} = \frac{A}{B}\times \frac{B}{C}\times \frac{C}{D}
= \frac{3}{4}\times \frac{5}{7}\times \frac{8}{9}
=> \frac{10}{21} = 10 : 21
Hence option [A] is the right answer.

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