We need to find how much 5 oranges weighs by multiplying 80 by 5.
80x5=400
So 5 oranges weighs 400 grams. We also need to find the weight of one grapefruit. Since 5 oranges, or 400 grams, weigh the same as 2 grapefruits, we need to divide 400 by 2.
400/2=200
So each grapefruit weighs 200 grams.
---
hope it helps