Hi, I have a question for which the answer is provided but I don't know how it is derived, could you help to explain it step by step please?

Thank you in advance!

The Sons of Knute had a hunting lodge up on Loon Lake that burned down last winter. They plan to rebuild it this summer and are trying to decide how large the new lodge should be. The organization has 50 members. The marginal rate of substitution of each of them between square feet of hunting lodge and money for other goods is 1.2 - 0.0004y, where y is the size of the hunting lodge in square feet. What is the efficient size for the new hunting lodge? Correct! O 2,000 square feet 1,200 square feet O None of the above. 1,000 square feet O 2,400 square feet