The Student Room Group

Maths percentages

For maths percentages how do you know when to use which? For example, a car increased by 10%.
How do I know when to times by 1.1 or divide by 0.9???
I keep get mixed up between these sort of things!
Reply 1
If u are told the original price was increased by 10% and now u want to find the original. Let P be the original price, then increasing by 10% would be 1.1*P, and so to find P from the increased price, u would divide by 1.1.

U divide by 0.9 if it was a 0.9 decrease to the original price. So: decreases of 10% would be 0.9P, so to find P from the decreased price, u divide by 0.9.
i don't understand division, but i interpret multiplying a price times 1.1 as 100% and 10/100 of the price
Reply 3
Original post by beautifulworld
i don't understand division, but i interpret multiplying a price times 1.1 as 100% and 10/100 of the price


yes if its a 10% increase, it will be 1.1x of the original. so to get back to the original, u do the opposite of multiplying by 1.1, which is dividing by 1.1.
Reply 4
Original post by dmedz
For maths percentages how do you know when to use which? For example, a car increased by 10%.
How do I know when to times by 1.1 or divide by 0.9???
I keep get mixed up between these sort of things!

Make sure you're completely comfortable with the backwards and forwards logic of what's going on.

As ignorabimus says, an increase of 10% corresponds to a multiplier of 1.1, so new price = 1.1 x old price and old price = new price / 1.1
A reduction of 10% corresponds to a multiplier of 0.9, so new price = 0.9 x old price and old price = new price / 0.9.

But - a reduction of 10% is not the "opposite" of an increase of 10%. This catches a lot of people out :smile:

Quick Reply