This problem can be solved by creating a system of equations and then solving that system.
x + y = 90
65x + 70y = 6000
The first equation is created from the combined total of the weight, and the second is created from the number of boxes being carried as well as the total weight of the truck. The variable x represents the number of large boxes, the variable y represents the number of small boxes. We can use substitution to solve the system.
x + y = 90
x = 90 - y
We have x equal to something, we can sub this into the second equation where there is an x.
65x + 70y = 6000
65(90 - y) + 70y = 6000
5850 - 65y + 70y = 6000
5850 + 5y = 6000
5y = 150
y = 30
Lastly, we can take the solution and sub it into the first equation to find x.
x + 30 = 90
x = 60
So, the large boxes weigh 60 pounds and the small boxes weight 30 pounds.