15 liters increased by 60 percent

15 * 60/100 = 9 liters

15 + 9 = 24liters

Initial quantity given in the question = 15 liters

Percentage of increase in the initial quantity = 60%

Then

Amount of increase in the initial quantity = (60/100) * 15

= (6/10) * 15 liters

= 90/10 liters

= 9 liters

Then

Total quantity after increase = 15 + 9 liters

= 24 liters

So the total quantity after increase of 60% becomes 24 liters. I hope the procedure is simple enough for you to understand. You can use this method in future for solving similar type of problems and you will surely get to the perfect and desired result. Only be careful about the calculation part.