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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?

This is a simple choosing problem.

15 choose 2 can be expressed as:

\(\binom{15}{2}=\frac{15\cdot14}{2\cdot1}=105\)

i hope this helped,

Gavin

This is just combinations, 15C2- \(\frac{15!}{2!*13!}=\frac{15*14}{2*1}=15*7=\boxed{105}\)