up to = less than or equal to
at least = greater than or equal to
The answer is correct
the max weight shall be 80
mass of small box = 5 lb. ( and there are x number of boxes )
Mass of big box = 12 lb. ( and there are y number of boxes )
adding them up shall be less than or equal to 80 ( as the shelf can withstand up to 80 lbs. )
And the minimum number of boxes shall be equal to 4
If we take the x as the number of boxes, then x would be greater than or equal to 4
Hope my answer helps!
