11.99 ≈ 12
24.99 ≈ 25
Round up the cost of the DVD is higher so that you down under estimate the cost.
100 - 24 = 75
Minus off the value of the DVD so you know what is the remaining money that you can spend.
75 ÷ 12 = 6 R 3
6 being the most number of CDs you can buy.
So, hey presto! You get the answer to your question and can watch a movie marathon too.