New
New
Finding the original amount after an increase
I can calculate the original value, given the final value after a stated percentage increase.
Lesson 9 of 16
New
New
- Year 8
Finding the original amount after an increase
I can calculate the original value, given the final value after a stated percentage increase.
Lesson details
Key learning points
- A bar model can be used to find the original amount after a percentage increase
- A double number line can be used to find the original amount after a percentage increase
- A ratio table can be used to find the original amount after a percentage increase
- In all of these representations a single multiplier can be used to find the original amount after a percentage increase
Keywords
Reciprocal - The reciprocal is the multiplicative inverse of any non-zero number. Any non-zero number multiplied by its reciprocal is equal to 1.
Common misconception
A single digit percentage is incorrectly worked out by dividing by 10 and not 100 e.g 3% = 0.3. This error continues when increasing amounts e.g increase 40 by 3% has a multiplier of 1.3
Remind pupils that to covert a percentage into a decimal we divide by 100. This applies with increase too e.g 120% has a multiplier of 1.2
After the slides on the traffic lights, use MWBs and change various times for the different parts or get the pupils to choose their own and fill in the other parts based on this. By asking them to choose their own you give them the opportunity consider appropriate values that work with time.
Teacher tip
Licence
This content is © Oak National Academy Limited (2025), licensed on Open Government Licence version 3.0 except where otherwise stated. See Oak's terms & conditions (Collection 2).
Lesson video
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Prior knowledge starter quiz
6 Questions
Q1.A multiplier is the decimal equivalent of a __________.
A multiplier is the decimal equivalent of a __________.
fraction
ratio
Q2.Match each question to the correct multiplier.
Match each question to the correct multiplier.
Q3.Which of the following is the correct calculation for calculating 360 increased by 5%?
Which of the following is the correct calculation for calculating 360 increased by 5%?
0.5 × 360
0.95 × 360
1.5 × 360
Q4.240 increased by 2% is .
240 increased by 2% is .
Q5.240 decreased by 20% is .
240 decreased by 20% is .
Q6.A box of washing powder contains 20% extra free. The box usually contains 6.5 kg. The special offer box contains kg.
A box of washing powder contains 20% extra free. The box usually contains 6.5 kg. The special offer box contains kg.
Assessment exit quiz
6 Questions
Q1.‘Of’ can be exchanged for which operation in maths?
‘Of’ can be exchanged for which operation in maths?
Add
Divide
Subtract
Q2.A number is increased by 20%. One part is 20%. What is the total number of parts representing the new number?
A number is increased by 20%. One part is 20%. What is the total number of parts representing the new number?
4
5
7
Q3.The cost of a TV increases by 15%. The new price is £460. What was the cost of the TV before the price increase? Without calculating the answer, decide whose answers cannot possibly be correct?
The cost of a TV increases by 15%. The new price is £460. What was the cost of the TV before the price increase? Without calculating the answer, decide whose answers cannot possibly be correct?
Alex says the answer is £391
Sofia says the answer is £400
Q4.Alex thinks of a number and increases it by 5%. His new number is 252. What was Alex's original number?
Alex thinks of a number and increases it by 5%. His new number is 252. What was Alex's original number?
239.4
264.6
265.26
Q5.The cost of a TV increases by 15%. The new price is £460. The cost of the TV before the price increase was £ .
The cost of a TV increases by 15%. The new price is £460. The cost of the TV before the price increase was £ .
Q6.This year the cost of a concert ticket is £94.50. This is an increase of 26% on last year. The cost of the ticket last year was £ .
This year the cost of a concert ticket is £94.50. This is an increase of 26% on last year. The cost of the ticket last year was £ .
