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.
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