Please help. Please tell me the answer and step by step.

In Euclidean geometry, two objects are similar if they have the same shape, or one has the same shape as the mirror image of the other. More precisely, one can be obtained from the other by uniformly scaling, possibly with additional translation, rotation and reflection so figure A and B are similar.
First step is finding the ratio, the a:b ratio is: 10/12.5=8/10=4/5
A:B=>4:5
16:x = 4:5
So x=20