We can write an equation to resemble the problem.
1160 = 170 + 50x
x equals the number of boxes he is carrying.
Solve for x.
1160 = 170 + 50x
990 = 50x
19.8 = x
We can't round up because that will be to much weight. So, we are going to round down.
19.8 -> 19
Therefore, the answer is 19.
Best of Luck!