1/5 of 15
There are lots of ways of thinking about this, and it's all about finding the way that makes sense to you.
One way is: 1/5 of 15 is the same as saying 15 multiplied by 1/5
i.e. 15 x (1/5) = (15/1) x (1/5) = (15 x 1) / (5 x 1) = 15/5 = 3
Another way is to just think about how many fives there are in fifteen. To do this all you need to do is go through your 5 times table.
i.e. 5 x 3 = 15 , so there are 3 fives in fifteen.
You'll most probably prefer the second way for simple problems but when you move onto harder problems you might find that that first method becomes more helpful when dealing with fractions.