in a US gallon, there are approximately 3.79 Liters, in a Liter there are 1000 milliLiters, so 3.79 Liters is then 3.79 * 1000 milliLiters, namely 3790 milliliters.
since a gallon is then approximately 3790 mL, how many times does 5mL go in to 3790? 3790 ÷ 5 = 758.
since the child uses 5mL in 1 day, then 758 times that is 758 days.