Abe had 9 liters of lemonade to sell. He sold 7 glasses with 550 milliliters per glass. How many milliliters of lemonade did he have left?

To solve problems with mixed units, we want to convert all units in the problem to one unit. Since the answer is asking for milliliters, we should convert all units in the problem to milliliters. One liter is 1000 milliliters, so Abe has 9 * 1000 = 9000 milliliters of lemonade.

Since Abe sold 7 glasses of lemonade, where each glass is 550 milliliters, he sold 7 * 550 = 3850 milliliters of lemonade total. That means he has 3850 milliliters of lemonade less than before. That means Abe has 9000 - 3850 = 5150 milliliters of lemonade left.