80 adult, 50 child. Got answer by trial and error, so no clue as to how to get the answer using any formula.
Account balance is not proportional to the number of weeks.
Assuming that you mean 7500 a year, Alex would have to pay 30,000 over the course of 4 years. His scholarship will bring that total down to 29,750. If he has four years to save, each year he should be saving $7,437.50. (29,750/4). For each month, he should be saving a total of $619.79 (7,437.50/12)