We need 15 liters
2 gallon jug 4.28*2=8.56  1 gallon=3.78, 2 gallons=7.57 liters
3 gallon jug 5.92*3=17.76             3 gallons=11.35 liters
4 gallon jug 6.56*4=26.24            4 gallons=15.14 liters
the best solution is to buy 2 times the 2 gallon jug because he will spent
8.56*2=17.12Â for 4 gallons(aprox 15 liters)
instead of the 4 gallon jug which costs 26.24
7 gallon this is more expensive and he does not that much solution