Hi Steve,

When you say "deducting 10% from £100" I assume you mean "deducting 10% of £100 from £100". It is normal in common language that we use the term percent without saying percent of what. Most times it is implicit but there are times when it is confusing.

10 % of £100 is 0.10 × £100 = £10, so deducting 10% of £100 from £100 is subtracting £10 from £100 which leaves £90.

Your second calculation relates to a different question. Suppose you had some unknown amount, you added 10% of that amount and the answer is £100. What amount did you start with? The answer is £90.91. Try is add 10% of £90.91 to £90.91 and see what you get.

Penny