2.30

3. The essence of this problem is to study binary linear equations. Let the fish head be X and the fish body be Y.

X= 1/2*Y+4

Y=X+4

Bring the second formula into the first one, and the solution is X= 12.

Substitute X= 12 into the second formula and get Y= 16.

Then the weight of the fish head is 12Kg, and the weight of the fish body is 16Kg.

4+ 12+ 16=32

This fish weighs 32 kilograms.

The answer is for reference only.