Well you already know that 2 cubic feet of sand weighs 90 pounds, so 90 ÷ 2 = 45.
Now you know that 1 cubic feet of sand weighs 45 pounds. Now you need to figure out how much 5 cubic feet of sand will weigh. If each cubic feet of sand weighs 45 pounds you just multiply 45 × 5 = 225
So 5 cubic feet of sand will weigh 225 pounds.
Hope that helps you. :-)