A club has 15 members and needs to choose 2 members to be co-presidents.  In how many ways can the club choose its co-presidents?
To choose 2 co-presidents out of 15 members, the order does not matter, so I can use combinations instead of permutations. The formula for combinations is n choose r, or nCr, which is equal to n!/(r!(n-r)!), where n is the total number of items, r is the number of items chosen, and ! means factorial. In this case, n is 15 and r is 2, so I can plug them into the formula and simplify. 15 choose 2 = 15!/(2!(15-2)!) = 15!/(2!13!) = (15*14*13!)/(2*13!) = (15*14)/2 = 105. Therefore, there are 105 ways to choose 2 co-presidents out of 15 members. 