If, A : B = 7 : 9 and B : C = 5 : 4, then A : B : C is:

1. 7 : 45 : 36

2. 28 : 36 : 35

3. 35 : 45 : 36

4. none of these

Given,

A : B = 7 : 9 and B : C = 5 : 4

L.C.M of two values of B, that are 9 and 5 is 45.

$\Rightarrow \dfrac{A}{B} = \dfrac{7 \times 5}{9\times 5} = \dfrac{35}{45} \\[1em] \Rightarrow \dfrac{B}{C} = \dfrac{5 \times 9}{4 \times 9} = \dfrac{45}{36}.$

A : B : C = 35 : 45 : 36.

Hence, option 3 is the correct option.