mr lewis is going on a trip

Question

He has 11 books he hasnt read yet, but he wants to bring only 2 on the trip. In how many ways can he choose 2 books to bring on the rrip?

Answer 1

Use combination. In the calculator it will just be ₁₁C₂ = 55.

 

The long method is _nC_r = (n!)/[r!(n-r)!]; ₁₁C₂ = (11!)/[2!(11-2)!] = 55

 

*Why combination and not permutation?

 

Because the order doesn't matter. Whether you pick the Math book first and the Science book second OR you pick the Science book first then the Math book second. You are still carrying the same two books on the trip.