I don't see why you would need that, but I suppose you could try to fit that equation. All that is being asked is that at the beginning of 2012, 36% of the original 250 walleye are no longer in the lake. So, the number of fish in the lake is 250 minus the 36% of 250 = 250-0.36*250 = 250(1-0.36) = 250(0.64) = 64% of 250 = 160 walleye at the beginning of 2012.