First you must set up an equation. Since there are 6 pitchers and 12 glasses you put. 6p + 12g = 21. Then they give you what p is (5g) and you plug that in and solve for g.
6(5g) + 12g = 21
30g + 12 g = 21
42 g = 21
Divide 42 on both sides to isolate the variable g. You get g = 0.5
The answer is that the capacity of each glass is 0.5 liters.
