Total for 80 per acre = 80*20 = 1600
X is number of plants added
New number is now 80 + X
Each tree only brings 16 fruits now RIP
The total number of bushels = (80 + X ) * 16 = 80*16 +16 X
No more than 1600
1360 +16X has to be larger than 1600
16 X has to be larger than 225
X should be greater than = 13.28
X has to be integer
15 trees must be added
Yay!
Edit:
How is this calculus... or did you mean calculate.