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Percentage

Chapter 8 Study Notes · O-Level 4052

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Section 9 of 10
Contents▾
8.0Why Percentage Matters8.1Percentage, Fraction and Decimal8.2Percentages Greater Than 100% and Less Than 1%8.3Percentage of a Quantity8.4Expressing One Quantity as a Percentage of Another8.5Percentage Change8.6Reverse Percentage8.7Profit, Loss and Discount8.8GST, Service Charge and Commission8.9Summary

8.8GST, Service Charge and Commission

GST in Singapore is 9%9\% (from 1 January 2024) and is added to most goods and services. Restaurants usually add a service charge of 10%10\% first, then GST on that total.

Price including GST
Price including GST=Price before GST×1.09\displaystyle \text{Price including GST} = \text{Price before GST} \times 1.09

GST calculation

A restaurant bill before GST is $86\displaystyle \$86. Find the total amount payable including 9%\displaystyle 9\% GST.

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Total\displaystyle \text{Total}
=109% of $86\displaystyle = 109\% \text{ of } \$86
=1.09×86\displaystyle = 1.09 \times 86
=$93.74\displaystyle = \$93.74

Finding the pre-GST price (reverse GST)

A receipt shows a total of $218\displaystyle \$218 inclusive of 9%\displaystyle 9\% GST. Find the price before GST.

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The price before GST is 100%100\%, so the price including GST is 109%109\%.
109% of price\displaystyle 109\% \text{ of price}
=$218\displaystyle = \$218
1% of price\displaystyle 1\% \text{ of price}
=218109\displaystyle = \dfrac{218}{109}
100% of price\displaystyle 100\% \text{ of price}
=218109×100\displaystyle = \dfrac{218}{109} \times 100
=$200\displaystyle = \$200

Service charge then GST (combined restaurant bill)

A restaurant meal costs $120\displaystyle \$120 before charges. A 10%\displaystyle 10\% service charge is added first, then 9%\displaystyle 9\% GST is applied to that subtotal. Find the final bill.

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Service charge: subtotal\displaystyle \text{Service charge: subtotal}
=120×1.10\displaystyle = 120 \times 1.10
=$132\displaystyle = \$132
Then GST: total\displaystyle \text{Then GST: total}
=132×1.09\displaystyle = 132 \times 1.09
=$143.88\displaystyle = \$143.88
In one move: 120×1.10×1.09=$143.88service charge, then GST120 \times 1.10 \times 1.09 = \$143.88 \quad \scriptsize\textit{service charge, then GST}

An agent's commission is a percentage of the sales value.

Commission calculation

An agent sells a flat for $450 000\displaystyle \$450\,000. The commission rate is 2%\displaystyle 2\%. Find the commission.

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Commission\displaystyle \text{Commission}
=2% of $450 000\displaystyle = 2\% \text{ of } \$450\,000
=0.02×450 000\displaystyle = 0.02 \times 450\,000
=$9000\displaystyle = \$9000

Now you try

An agent sells a property for $200 000\$200\,000. The commission rate is 5%5\%. Find the commission earned.

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Commission =5%= 5\% of $200 000\$200\,000
=5100×200 000= \frac{5}{100} \times 200\,000
=$10 000= \$10\,000

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