To do this, we must work through the problem backwards.
Smith has 15 now, however the last thing he did was buy 10 (+10)
To go back a step we must do the opposite of add 10, subtract 10
15 - 10 = 5
Smith sold 4/5 of his DVDs to leave him with 5
This means that 1/5 = 5
1/5 of the original amount was equal to 5
To make 5/5 (or a whole) we multiply by 5
5 x 5 = 25
Smith has 25 DVDs to begin with