Answer:
69 trees must be there this year to get the maximum yield.
Step-by-step explanation:
Let the number of trees = x.
For 90 trees, the total yield is 80 bags.
That is, the yield per tree = 
For 80 trees, the total yield is 85 bags.
That is, the yield per tree = 
So, For every decrease in 10 trees, the yield per trees increases by 1.06-0.88 = 0.18.
Thus, yield per tree, 
i.e. Yield per tree = 
Then, the total yield, T = Number of trees(x) × Yield per tree(Y)
i.e. Total yield, T = 
i.e. Total yield, T = 
Now, to maximize the total yield, we will differentiate T with respect to x and equate to 0.
We get,
implies 
i.e. 
i.e. x= 69
Thus, 69 trees must be there this year to get the maximum yield.