How many ways can you buy 2 DVDs from a display of 15?
We assume that someone will not buy two of the same DVD.
For the first DVD, there is a choice of 15 , but once the first is bought, there is a choice of 14 for the second one.
However, this does not take order into consideration .... the 2 DVD's might be the same, just bought in a different order, so we need to divide this number by 2 to avoid the duplicates.
This is the condition of 'combinations'