Question 1203933
x = number of large boxes
120-x = number of small boxes

40x = weight of the large boxes only
25(120-x) = weight of the small boxes only
40x + 25(120-x) = total weight = 3750
40x + 25(120-x) = 3750

Let's solve for x.
40x + 25(120-x) = 3750
40x + 3000-25x = 3750
15x + 3000 = 3750
15x = 3750-3000
15x = 750
x = 750/15
x = 50
120-x = 120-50 = 70

Answers:
50 large boxes and 70 small boxes