By the time all the milk is gone, how much water has the milkman sold?

Question

A milkman has a can of 20 gallons, full of milk. But due to a sudden epidemic, all his cows in the farm fall sick. He can no longer collect milk from them.
To conserve his milk he decides that on the first day he will sell one liter and refill the can back with water. On the 2nd day he will take out 2 liters and refill the bottle. On the 3rd day he will take out 3liters and so on...
By the time all the milk is gone, how much water has the milkman sold?

Answer 1

2850 liter
He starts with 20 gallon = 75.7 liter.
On the 76th day he will sell the entire content of the can. Only then will he have sold all the milk. He will also have sold all the water he has added.
He has added 1 + 2 + 3 + ... 74 + 75 liter water = 2850 liter water.