If I understand the problem, the Yield Y looks like this on a trending basis
Y = (100)(150)
= (100+1)(150-1)
= (100+2)(150-2) so the general trend will be if E is the number of extra trees
= (100+E)(150-E) = 15,000 + 50E - E2
Let's find the max yield, dY/dE = 50 - 2E =0 implies E = 25
This means that if you plant 25 extra trees, the Yield will be maximum at 15,625.
If you double the number of trees then, Y = (100+100)(150-100) = 10,000.
This should be enough information to answer all questions!