Math CentralQuandaries & Queries


Question from Steve:

What is the correct way of deducting 10% from £100?
Would it be £100 multiplied by 0.1 and then deduct to leave you £90; or
Would it be £100 divided by 1.1 to give you £90.91?

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.


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