A. 20%

B. 25%

C. 28%

D. 30%

__Solution(By Examveda Team)__

Let the initial total weight of A and B be x kg.

Then,

$$\eqalign{ & = 125\% {\text{ of }}x = 80 \cr & \Rightarrow x = 80 \times \frac{{100}}{{125}} = 64{\text{kg}} \cr & {\text{A's initial weight}} \cr & = \left( {64 \times \frac{3}{8}} \right){\text{kg}} \cr & = 24{\text{kg}}{\text{}} \cr & {\text{B's initial weight}} \cr & = \left( {64 \times \frac{5}{8}} \right){\text{kg}} \cr & = 40{\text{kg}} \cr & {\text{A's new weight}} \cr & = 120\% {\text{ of }}24{\text{kg}} \cr & = 28.8{\text{kg}}{\text{}} \cr & {\text{B's new weight}} \cr & = \left( {80 - 28.8} \right){\text{kg}} \cr & = 51.2{\text{kg}} \cr & {\text{Increase in B's weight}} \cr & = \left( {51.2 - 40} \right){\text{kg}} \cr & = 11.2{\text{kg}} \cr & \therefore {\text{Increase }}\% \cr & = \left( {\frac{{11.2}}{{40}} \times 100} \right)\% \cr & = 28\% \cr} $$