Tom gets $12 off a box of chocolates that had an original priceof $48. what percentage is the discount?

Well you can just start off by doing 48/12 which is 4. So he got 1/4 off the box of chocolates, and 1/4 in percentile form is 25%. 
He got a discount of 25%

One way to calculate this is finding what the percentage of 36 is out of 48.
 
36/48= 0.75

You then have to subtract this number from 1 (since you're asking for a markdown, not up) and get 0.25.

0.25 x 100 = 25%

The discount is therefore 25%

Alternatively, you could subtract 36 from 48 to get 12, the amount it was marked down by. Then find the percentage of 12 out of 48.

12/48 = 0.25

0.25 x 100 = 25%

Hope this helps :)