Tank X = 3(Tank Y)
Tank X = W(water) + 56(liters)
Tank Y = W(water) + 16(liters)
Therefore: W + 56 = 3(W + 16)
W + 56 = 3W + 48
-W = -W
56 = 2W + 48
-48 = -48
8 = 2W
4 = W
Tank X = 4 + 56 = 60 liters
Tank Y = 4 + 16 = 20 liters
4 liters of water is needed to fill both tanks.