There are two ways to do this, the one that makes more logical sense and the faster one
You can first calculate how much is 25%. You do this by $24.99 × 25 ÷ 100
Since you know how much is "discounted", just minus the answer you get above with the original price ($24.99 - $6.2475)
The quicker way is to recognise that "original price" is always 100%. In other words, you can just do 100% - 25% and get 75%. Then, $24.99×75÷100 is your answer.