Permutation questions- urgent help needed

DrSteve
Wed, 23 Jan 2013 11:09:21 +0000

For the plumbing problem, there are 4C1 ways to choose 1 experienced plumber from 4, and 4C2 ways to choose 2 trainees from 4. By the counting principle, the answer is (4C1)(4C2) = (4)(6)=24.

Khan’s mistake in the video is that he is using permutations instead of combinations. Selecting trainee E followed by trainee F is the same as selecting trainee F followed by trainee E. His math doesn’t account for this which is why his answer came out to double the correct answer.