How many ways can four letters abcd be arranged such that a always comes before b and c always comes before d?
Total number of ways abcd can be arranged? 4! Half of them a if before b, half b is before a. Similarly, in half, c is before d, and other half, d is before a.
But we can't divide 4! by 2 - because there are some in which b is before a AND d is before c. So what's the answer?