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

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

1. 5 : 3
2. 3 : 5
3. 7 : 21
4. 7 : 15

Correct Option: A

a : c = (a : b) × (b : c)

=	7	×	15	=	15	= 5 : 3
	9		7		9

Second Method :
A : C = 7 × 15 : 9 × 7 = 5 : 3