Please write the whole question, as it tends to avoid confusion and you stand a much better chance of getting your question answered too.
First, what is 17.5% of 50? Whatever it is, we will take it away from the 50.
So, is the fractional version of the percentage: PERCENT --> PER CENT --> out of 100.
And as "of" means multiply, we get --> £8.75.
So, 50-8.75 is £41.25.
Quick method: 0.825*50 is 41.25. Done. Issue is how did i get the 0.825 from just looking at the question? Answer... read my Lessons on this site on Percentage Increase and Percentage Decrease. They will help you.