SOLUTION: A shelf has 7 probability books and 9 electonics books. Find the probability that 5 particular probability books will be together.

Altogether there are 16 books.

There are 5 particular probability books and 11 other books.

Think of making a "book bundle" by tying a rope around the 5 
particular probability books.

Then we have 12 "things" to arrange, 11 other books and 1 
"book bundle".

There are 12! ways to arrange those 12 "things" on the shelf.

But there are 5! ways the 5 particular probability books can
be arranged within the "book bundle".

So the number of ways to arrange the books successfully is 12!5!  

That's the numerator of the desired probability.

The denominator of the desired probability is the number of ways 
the 16 books can be arranged on the shelf, which is 16!

So the desired probability is , which simplifies
to .

Edwin