Answer: 80 trees
Explanation:
YIELD = (NUMBER OF TREES)*(NUMBER OF ORANGES PER TREE)
Let's assume NUMBER OF TREES = 60 + x, where x is the number of additional trees above 60
The NUMBER OF ORANGES PER TREE will = (400-4x). Hence:
YIELD = (60+x)*(400-4x) = 24000-240x+400x-4x2 = -4x2 + 160x + 24,000
To find the maximum YIELD, take the derivative of YIELD wrt x, set it to zero, and solve for x:
d(YIELD)/dx = -8x + 160
0 = -8x +160
8x = 160
x = 20
The grower should grow 60 + 20 = 80 trees to maximize yield.