Question 171758
A musician plans to perform 4 selections. In how many ways can she arrange the musical selections? 
4 * 4 = 16
4 * 3 = 12
4 * 2 = 8
4 * 1 = 4 
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Assuming she doesn't do any repeats:
For the 1st, he can choose 1 of 4.
For the 2nd, 1 of 3
Then 1 of 2
So, it's 4*3*2*1 = 24
That's called 4 factorial, and is written 4!
Since the order matters, that's the answer, 24.