Secondary 3 G2 Mathematics Tutor Bukit Timah | Reverse Percentages and Original Prices. Did you know? A discounted price is not the starting price.
This quick-reference guide explains reverse percentages and original prices for Secondary 3 G2 Mathematics students in Bukit Timah, with worked examples, mistakes to avoid and independent practice.
Start with the multiplier
A 20% discount leaves 80% of the original price, so sale price = 0.8 × original price. To recover the original, divide the sale price by 0.8.
Try explaining this step in your own words before checking a second example. A useful Mathematics tutorial checks the first incorrect decision, then changes the numbers or diagram to test whether the method transfers.
Worked discount
A hypothetical sale price is $72 after 20% off. Original price = 72/0.8 = $90. Check: 20% of $90 is $18, and $90−$18=$72.
Try explaining this step in your own words before checking a second example. A useful Mathematics tutorial checks the first incorrect decision, then changes the numbers or diagram to test whether the method transfers.
Reverse an increase
A fictional item costs $138 after a 15% increase. Original price = 138/1.15 = $120. The increase is $18.
Try explaining this step in your own words before checking a second example. A useful Mathematics tutorial checks the first incorrect decision, then changes the numbers or diagram to test whether the method transfers.
Successive changes
A $100 item discounted by 10% and then 20% costs 100×0.9×0.8=$72. The combined discount is 28%, not 30%.
Try explaining this step in your own words before checking a second example. A useful Mathematics tutorial checks the first incorrect decision, then changes the numbers or diagram to test whether the method transfers.
Different percentage bases
An increase from $80 to $100 is 25% of $80; a fall from $100 to $80 is 20% of $100. Both are correct with different bases.
Try explaining this step in your own words before checking a second example. A useful Mathematics tutorial checks the first incorrect decision, then changes the numbers or diagram to test whether the method transfers.
Self-check
If an item is $54 after a 10% discount, original price = 54/0.9 = $60. Multiply the answer by 0.9 to verify.
Try explaining this step in your own words before checking a second example. A useful Mathematics tutorial checks the first incorrect decision, then changes the numbers or diagram to test whether the method transfers.
Frequently asked questions
What should I do first? Identify the given values and the quantity required. How can I check the result? Substitute it into the original information and verify the units or graph. Why might a correct-looking answer fail? A sign, denominator or interpretation error can change the meaning of the result.
Official syllabus and tutorial context
Consult the 2027 SEC G2 Mathematics K210 syllabus for the official topic and assessment scope. The unchanged eduKateSG three-pupil Mathematics reference describes small-group teaching near Sixth Avenue. Confirm current Secondary 3 G2 tutor, fees, venue, group fit and timetable directly.
Continue the Secondary 3 G2 Bukit Timah series
For further Mathematics revision, see factorisation by grouping, quadratic graphs, and critical reading of data.