If a : b = c : d then, (ma + nc) / (mb + nd) is equal to?

Let (a/b) = (c/d) = k
Then, a = bk and c = dk.
∴ (ma + nc) / (mb + nd) = (mbk + ndk) / (mb + nd) = k(mb + nd/mb + nd)
= k = a/b = a : b.