What are reverse percentages?
In a reverse percentage question, we are given an amount that has recently increased or decreased by a certain percent.
We must then use this information to find out what the original amount was.
Examples
- The price of a t-shirt has decreased by 10% in a sale. It is now selling for £18.
What was the original price of the t-shirt? - The price of a TV has increased by 20% and is now £240. How much was the TV originally?
The first example here involves a decreased amount, whereas the second example asks about an increased amount.
How can we work out the answers to these questions?
How to find the original amount after a percentage decrease
Example question
The cost of a t-shirt has been decreased by 10% in a sale.
Its new price is £18.
What was its original price?
Step 1
We know that the t-shirt is now at 90% of its original value, because it started at 100% of its value and decreased by 10%.
So, 90% of the original price = £18.
Step 2
If we can work out 10% of the amount, we can then multiply this by 10 to find 100%.
If 90% = £18, then £18 divided by 9 will give us 10%.
£18/9 = £2
Step 3
So, 10% = £2. We can multiply this by 10 to find 100%.
£2 x 10 = £20.
In summary, we have found that:
90% of the original amount = £18
10% of the original amount = £2
100% of the original amount = £20.
The answer is £20!
Top Tip
We could easily find 10% of the amount in this question. For other questions, it may well be necessary to find 1% of the original amount instead, and then multiply this by 100 to find 100%.
In the above example, we could have done this by dividing 18 by 90, to find 1%, and then multiplying 0.2 by 100 to reach 20.
How to find the original amount after a percentage increase
Example question
The price of a TV has increased by 20% and is now £240.
How much was the TV originally?
Step 1
We know that the TV is now at 120% of its original value, because it started at 100% of its value and the question tells us that it increased by 20%.
So, the 120% of the original price = £240.
Step 2
If we work out 10% of the original price, then we can find 100% of the original price.
If 120% = £240, we can divide £240 by 12 to find 10%.
220/12 = 20.
Step 3
So now we know that 10% = £20.
If multiply £20 by 10, we will find the original price of the TV, as this will be 100% of the original value.
20 x 10 = 200.
Final answer: the original price of the TV, before it was increased in price, was £200!
Top Tip
We could also have worked this out by first finding 1% of the original price, and then multiplying this by 100 to find 100%.
In the above example, we could have done this by dividing 240 by 120, to find 1%, and then multiplying 2 by 100 to reach 200.
Further Practice
For questions on reverse percentages in expert 11+ Maths practice exam papers, take a look at the papers in our shop:
