Diego brought 2 1/2 liters of punch to a party. After the party, 3/8 of a liter of punch was left. How much punch was used at the party?
To find the amount of punch used at the party, subtract the amount left from the amount brought: 2 1/2 - 3/8 = 2.5 - 0.375 = <<2.5-0.375=2.125>>2.125 liters.
Diego used 2.125 liters of punch at the party. Answer: \boxed{2.125}.