500
                                Carlos has a large box of donuts with enough donuts to last 21 days.  If Carlos eats only one donut every morning, how many weeks will the box of donuts last?
A  1 week, because 21 ÷ 21 =1   
B 21 weeks, because 21 ÷ 21 =1 
C  3 weeks, because 21 ÷ 7 = 3
D  7 weeks, because 21 ÷ 3 = 7
                                What is 3 weeks because 21 ÷ 7 = 3  (C) ?