We need to find the weight of the food and water. To do this, find the difference between his backpack before the food and water and after. His backpack without the food is 7 3/4 pounds, and his backpack after is 11 3/4 pounds.
11 3/4 - 7 3/4
4
The food and water together weigh 4 pounds. We know the water weighs 1 3/4 pounds, so we can subtract it from the total to find what the food weighs.
