15. The staff of a company work 5 days and rest 2 days every week, and the company requires that there must be at least 45 people working every day from Monday to Sunday. Therefore, the company needs at least ( ) staff members.
【Analysis】Choosing 2 days off from Monday to Sunday, there are $C_{7}^{2}=21$ (ways) in total. The best arrangement is to have 21 people each choose a different way, 
so that every day $21-(7-1)=15$ people are at work,