15$ is the amount that was taken off so now we just need to convert it to a % of 45
45/100 = 30/?
if you do cross multiplication and multiply 30 * 100 then divide the result (3000) by 45 you get 66
This means that 30$ is 66% of 45. To find the discount you just subtract 66% (this represents your 30$ new price) from 100% (this represents the 45% original price) and you get 33%.