If the mass of a is 5.0 g and the mass of b is 80g what are their relative masses

The word "Relative" in the problem means, actual / total.

Actual mass of A = 5 g Total mass = (5+80) g = 85 g

Relative masses are ratio of the masses. 80 g of A / 5.0 g of B = a ratio of A: B of 16: 1. Therefore the relative mass of B is 16 times that of A.