5 objects are arranged in a line. What is the probability that two of them, determined before, are next to each other:
Just tie those two together
so now you have 4 objects
they can be arranged in 4! ways. but of the 2 tied together, either could be first so there are 2 ways those 2 can be arranged.
Altogether there are 2*4! ways to arrange them
But if there were no restrictions then it would be 5! ways.
Can you finish it?