Chapter 2 - The Key Principles of Economics - Exercises - 2.3 The Principle of voluntary Exchange - Page 45: 3.6a

50 fish

If the boat builders are chosen at random, it can be expected that half will come from the group that catches 2 fish and half will come from the group that catches 8 fish. The opportunity cost of building the boat is therefore $(5\times2)+(5\times8)=10+40=50\ $fish