Answer:
To find out how much Gary's backpack will weigh with the math and science textbooks in it, we need to add the weight of the textbooks to the weight of the backpack:
Total weight = backpack weight + math textbook weight + science textbook weight
Total weight = 1.2 + 3.75 + 2.85
Total weight = 7.8 pounds
Therefore, Gary's backpack will weigh 7.8 pounds with the math and science textbooks in it.