For interesting enrichment in your mathematical thinking about this problem, 
let's look at the "no-algebra" way to do it -- the only way you could have done
it when you were in elementary school!  Some elementary school teachers have
labeled it the "WHAT IF?" method. 

If all 120 boxes had weighed only 25 pounds each, the truck 
would be carrying only (120)(25) = 3000 pounds.  So some of 
the 120 boxes must each weigh 40-25 = 15 pounds heavier to 
cause the cargo to weigh 3750-3000 = 750 pounds heavier. So 
there must be 750/15 = 50 large boxes, and 120-50 = 70 small 
boxes.

Or you could go this route instead without using algebra:

If all 120 boxes had weighed 40 pounds each, the truck would 
be carrying (120)(40) = 4800 pounds.  So some of the 120 
boxes must each weigh 40-25 = 15 pounds lighter to cause the 
cargo to weigh 4800-3750 = 1050 pounds lighter.  So there 
must be 1050/15 = 70 small boxes, and 120-70 = 50 large boxes.

Edwin