Discount, then GST, and the second percentage is of the new price is in the Primary 5 Percentage chapter. The lesson film below teaches it with its own questions, and the practice set is written for Primary 5.
Lesson video, with the working
Each price has its own 100 percent
3 min 18 s
The film teaches 2 questions in turn. Each question shows first; its working is written as the film explains it.
A television has an original price of $500. The shop gives a 20% discount, and then 9% GST is added to the discounted price. How much does the customer pay?
The customer pays $436.
Ans: $436
Example 1
A ceiling fan is sold at a 20% discount. After the discount it costs $240. What was the original price of the fan?
The fan's original price was $300.
Ans: $300
How to solve it
The original price, 500 dollars, is 100 percent. A 20 percent discount leaves 100 percent minus 20 percent, which is 80 percent to pay.
Find 1 percent: 500 dollars divided by 100 is 5 dollars. So 80 percent is 80 times 5 dollars, which is 400 dollars.
GST is charged on the discounted price, so 400 dollars is the new 100 percent.
Add 9 percent to get 109 percent. 1 percent is 4 dollars, so 109 percent is 109 times 4 dollars, which is 436 dollars.
Working back from a discounted price: if 80 percent is 240 dollars, 1 percent is 3 dollars, so the original price is 300 dollars.
Always identify the price that represents one hundred percent before calculating.
Practice questions with worked solutions
Question 1 is free to try. The other 10 open in the free trial, each with its full working.
How do you work through P5 discount, GST and interest questions?
Discount, GST and annual interest are where Primary 5 percentage meets shopping and saving.
What your child learns here
Discount, GST and annual interest are where Primary 5 percentage meets shopping and saving. This page uses one method throughout: the whole stands for 100%, so 1% is the whole divided by 100, and your child learns which amount is the whole at each step.
A discount is a percentage of the original price, and the customer pays the rest: 20% off $500 means paying 80%, which is $400.
GST is a percentage of the price before GST, so after a discount it is worked on the discounted price: 109% of $400 is $436.
A percentage discount comes from the amount saved: a chair sold for $540 after $60 off had an original price of $600, and 60/600 is 10%.
Annual interest is a percentage of the amount deposited, earned over one year.
The same 1% step, the whole divided by 100, works for all three.
Does the answer make sense?
A price after a discount must come out below the original price. Run Question 1 a second way: 25% of $300 is $75, and $300 minus $75 is $225, the same amount as 75% of $300.
How to practise these questions
Free Question 1 takes 25% off a $300 school bag and asks what the customer pays. Basic questions train finding a discount, the price after a discount, 9% GST added to a price, and the interest a deposit earns in one year. Intermediate questions put a discount and then GST on the same item, one step after the other. Challenge questions find the percentage discount from the price paid and the amount saved, add a service charge before GST, give the amount in the bank after one or two years of interest, or price a repair job or a poster order with a percentage step inside it. If the whole keeps changing in the wrong place, open its worked solution and try the question again later from Revision.
Questions parents ask
Why not just subtract 20% and add 9%? Because the two percentages are of different wholes. The 20% is of the original price and the 9% is of the discounted price, so the steps are done one at a time, in order.
Which GST rate is used? Each question states its rate. The lesson and the GST practice questions use 9%, Singapore's current rate.
Is annual interest in Primary 5? Yes. Finding discount, GST and annual interest is one Primary 5 Percentage outcome. Most interest questions here cover one year, and one Challenge question carries the interest into a second year.
