Which of these numbers is a prime number 1,11,21,51

Asocial networking site currently has 400,000 active members, and that figure is predicted to increase by 32% each year. the following expression represents the number of active members on the site after t years.

From a group of 10 men and 8 women, 5 people are to be selected for a committee so that at least 4 men are on the committee. howmany ways can it be done?

Jack has to solve the problem 281-266.how would you tell him to solve the problem ? tell which strategy you would use ,and solve the problem.