Blue whale: 14 ton

This problem can be solved by a series of equations. First, suppose that the weight of an elephant is x tons, the weight of a cow is y tons, and the weight of a blue whale is z tons, then the following equation can be obtained:

X+5Y= 10

2X+5Y= 16

X+Z=20

Solution: X=6, Y=0.8, Z= 14.

I don't know if you have learned to solve equations, so please ask if you have any questions.