If a : b = c : d = e : f = 1 : 2, then (3a + 5c + 7e) : (3b + 5d + 7f) is = ?

Answer: Option D

a : b = 1 : 2
c : d = 1 : 2
e : f = 1 : 2
∴ a = x, b = 2x, c = x, d = 2x, e = x, f = 2x
$$\begin{array}{rl}& \therefore \frac{3\times x+5\times 7+7\times x}{3\times 2x+5\times 2x+7\times 2x}\times 1\\ & \Rightarrow \frac{15x}{30x}\\ & \Rightarrow \frac{1}{2}\end{array}$$