Plz

hello everybody... this is my first time in this forum, well i have this problem, can anyone kindly help me in solving it:
a police station will start making a night shift, and there are 20 policemen.

a.if a night team includes 3 policemen, how many different teams are possible to have??

b.if the policemen are ranked 1,2,...,20 in order of doing well, how many of these teams will not have the beat policemen??

c.How many the teams will have at least 1 of the best 10 policemen??

d.if one of these teams is selected at random to work in a particular night, what is the possibliy that the best policeman will not work that night??