If a : b = 7 : 9 and b : c = 15 : 7, then what is a : c?

If a : b = 7 : 9 and b : c = 15 : 7, then what is a : c?
[A]7 : 15
[B]3 : 5
[C]5 : 3
[D]7 : 21

5 : 3
a : c = (a : b) \times (b : c)
= \frac{7}{9}\times \frac{15}{7} = \frac{15}{9} = 5 : 3
Hence option [C] is the right answer.

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