Find liters per day by dividing the amount made by the number of days :
10 liters / 2 days = 5 liters per day.
Find how much more he needs to make by subtracting the amount made from the total he wants:
15 liters - 10 liters = 5 liters more needed.
Divide amount needed by time:
5/5 = 1
He will need to spend 1 more day, which means he will spend a total of 3 days to make 15 liters.
