单项选择题 To fill a number of vacancies, an employer must hire 3 programmers from among 6 applicants, and 2 managers from among 4 applicants. What is the total number of ways in which she can make her selection ？（）

单项选择题 A child withdraws from his piggy bank 10% of the original sum in the bank. If he must add 90￠ to bring the amount in the bank back up to the original sum, what was the original sum in the bank ？（）

单项选择题 Last year, an appliance store sold an average (arithmetic mean) of 42 microwave ovens per month. In the first 10 months of this year, the store has sold an average (arithmetic mean) of only 20 microwaves per month. What was the average number of microwaves sold per month during the entire 22-month period ？（）