Problem
COM-B1-M02-P012 One Before Another
#12
★★☆☆☆ Level 2 of 5
How many permutations of \(1,2,3,4,5\) have \(1\) before \(2\)?
For every permutation with \(1\) before \(2\), there is a paired one with \(2\) before \(1\).
There are \(5!=120\) permutations. By symmetry, exactly half have \(1\) before \(2\). Answer: \(60\).
Important distinction from adjacency.