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How do I make calculations with percentages?

There is a simple way of making calculations with percentages.

Say you want to subtract from a numerical value x,20%. Then multiply it with 80/100.

Say you want to add 20%.Then multiply with 120/100.

Say you want to add 70%. Then multiply with 170/100.

In short,if you want to add x%,then multiply with (100+x)/100. If you want to subtract y% then multiply with (100-y)/100.Note that you can't subtract more than 100%.

Easy,right? :P

Now we will look on a particular tricky question. We have a TV that costs 1000 pounds.Then,the price of TV is reduced with 20%.After some time,the TV is increased with 20%. What is the final price of a TV?

Don't fall into the trap of saying that the final price is the same with the initial one,therefore 1000 punds.

1.) We reduce with 20% : 1000*80/100=800 pounds

2.) We increase the current price with 20%,so: 800*120/100=960 pounds.

So the final price is 960 pounds.

Marco-Iulian G. GCSE Maths tutor, Uni Admissions Test .MAT. tutor, A ...

2 years ago

Answered by Marco-Iulian, a GCSE Maths tutor with MyTutor


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