GST, Discount and Annual Interest
Learn to apply percentages in real-life money situations!
1. Goods and Services Tax (GST)
GST stands for Goods and Services Tax
It is a tax on goods and services in Singapore. The current GST rate is 9%.
Formula:
GST = 9% × Price (before GST)
Total = Price + GST
Alternative:
Total = 109% × Price
(100% + 9% = 109%) For example, if the price is $100, Total = 109% × $100 = $109
Example: Mr Tan bought a fan for $80 (before GST). How much did he pay in total?
Method 1: Find GST first
GST = 9% × $80
= $80
= $7.20
Total = $80 + $7.20 = $87.20
Method 2: Find 1% first
100% of price = $80
1% of price = $80 ÷ 100 = $0.80
9% of price = $0.80 × 9 = $7.20
Total = $80 + $7.20 = $87.20
Example: A washing machine costs $500 before GST. What is the price including 9% GST?
GST = 9% × $500 = $500 = $45
Price with GST = $500 + $45 = $545
2. Discounts
A discount is a reduction in price
Discount = Percentage discount × Usual price
Price after discount = Usual price − Discount
Example: Jiahao bought a T-shirt at a 25% discount. The usual price was $36.
(a) How much was the discount?
Discount = 25% × $36
= $36
= $9
(b) How much did he pay?
Price after discount = $36 − $9
= $27
Alternative method:
100% − 25% = 75%, so he paid 75% × $36 = $36 = $27
Example: A camera costs $1,650. Mr Salleh bought it at a 20% discount. (a) How much was the discount? (b) How much did he pay?
(a) Discount = 20% × $1,650 = $1,650 = $330
(b) Amount he paid = $1,650 − $330 = $1,320
3. Annual Interest
Interest is money a bank pays you for keeping your savings with it
Interest for 1 year = Interest rate × Amount deposited (the principal)
Total after 1 year = Principal + Interest
Example: Mrs Li deposited $12,000 in Star Bank for one year at 1% interest per year. How much interest did she receive at the end of the year?
Interest = 1% × $12,000
= $12,000
= $120
Mrs Li received $120 in interest.
Example: Mr Siva deposited $26,000 at ABC Bank at 3% interest per year. How much will he have after one year?
Interest = 3% × $26,000 = $26,000 = $780
Total = $26,000 + $780 = $26,780
📌 Summary
| Type | Formula | Final Amount |
|---|---|---|
| GST (9%) | GST = 9% × Price | Price + GST (ADD) |
| Discount | Discount = Percentage discount × Usual Price | Price − Discount (SUBTRACT) |
| Interest | Interest = Interest rate × Principal | Principal + Interest (ADD) |
Practice Problems
Practice 1: GST
A tablet costs $450 before GST. How much is the total price including 9% GST?
ShowHide solution
GST = 9% × $450. 1% of $450 = $4.50, so 9% of $450 = 9 × $4.50 = $40.50
Total = $450 + $40.50 = $490.50
Practice 2: Discount
A bicycle has a usual price of $280. During a sale, it is sold at 15% off. How much does it cost during the sale?
ShowHide solution
Discount = 15% × $280. 1% of $280 = $2.80, so 15% of $280 = 15 × $2.80 = $42
Sale price = $280 − $42 = $238
Or: 85% × $280 = $238
Practice 3: Interest
Mdm Ong deposits $2,986 in her savings account for a year. She earns 2% interest per year. How much will she have at the end of one year?
ShowHide solution
Interest = 2% × $2,986. 1% of $2,986 = $29.86, so 2% of $2,986 = 2 × $29.86 = $59.72
Total = $2,986 + $59.72 = $3,045.72
Practice 4: More GST
A wallet costs $12 before GST. Doris bought it and paid 9% GST. How much did she pay for the wallet?
ShowHide solution
GST = 9% × $12. 1% of $12 = $0.12, so 9% of $12 = 9 × $0.12 = $1.08
Total = $12 + $1.08 = $13.08
Key Takeaways
- GST (9%): Tax added to price → Total = Price + GST
- Discount: Reduction from price → Final = Price − Discount
- Interest: Earned on savings → Total = Principal + Interest
- To find a percentage of an amount, write the percentage as a fraction over 100 and multiply, e.g. 9% of $80 = $80 = $7.20
- Alternative: Find 1%, then multiply by the percentage needed