There are 8 pints of milk in 1 gallon. There is also 4 quarts in 1 gallon. So each quart is 2 pints. After breakfast they had 3 quarts of the original gallon left since 4 - 1 = 3. 3 quarts is the same as 6 pints. They drank 3 pints at lunch. This means 6 - 3 = 3. After lunch, there were 3 pints left.
