Question

Paul's bathtub is clogged. He has to empty 30 liters of water by hand. Paul has a 3-liter, a 4-liter, and a 5-liter bucket. If Paul carries two buckets each trip, what combinations of sizes allow him to empty the bathtub in exactly four trips?

Answer 1

5 and 3 = 8 liters ,  5 and 4 = 9 liters, and 3 and 4 = 7 liters.

8 + 7 + 8 + 7 = 30 liters, so he can take the 5 liter bucket with the 3 liter bucket twice, and then use the 3 liter bucket and the 4 liter bucket twice.