· Built around the Discount calculator
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Most money mistakes are not about tax tables. They are about percentages: a "50% off" sign that is really 23%, a tip that quietly becomes 15%, a "9% a year" that means two very different things. Black Friday lands on 27 November and the year-end restaurant season straight after it, so this post works through the sums you will meet - with the percentage, discount, split the bill, simple interest and investment growth rate calculators.
Run your own numbers
The calculator starts on a stacked sale: R1 000 less 20%, then another 10% at the till. Change the price, or switch to "the % off" and type the sale price on the tag.
Checked against published examples
Before publishing, CalcSA ran the same inputs through its calculators:
- Simple vs compound: R10 000 at 9% a year for 10 years = R19 000 simple, R23 673.64 compound - Siyavula's Grade 10 Mathematics textbook (Finance and growth, "The power of compound interest"), written to the CAPS syllabus. Our simple interest calculator gives R9 000 interest, R19 000 at the end and R23 673.64 compounded yearly - the same to the cent.
- Compound growth: R30 000 at 6% a year for 4 years = R37 874.31 - Siyavula's worked example 5 on the same page. Run backwards through our growth rate calculator, R30 000 to R37 874.31 in 4 years gives 6.00% a year - it finds the rate the textbook started from.
- VAT inside a price: 15/115 of a VAT-inclusive price is the VAT - the tax fraction in SARS's VAT guidance. On an R800 sale price that is R104.35, which our VAT calculator also gives.
- Discounts, percentage change and tips are plain arithmetic - there is no official worked example to check against, so the sums are shown line by line below.
Worked example 1: what a sale is really worth
- R1 299 down to R999 is 23.09% off - R300 / R1 299 (discount calculator, "the % off" mode). Not the "up to 50%" on the banner.
- 20% off, then 10% more at the till: R1 000 x 0.8 x 0.9 = R720. That is 28% off. The second discount comes off the already-reduced price, so the two never add up.
- Of that R720, R93.91 is VAT - 720 x 15/115 (VAT calculator). Shelf prices in South Africa must include VAT, so the tag is what you pay.
- A price that rises 25% and then falls 25% does not come back: R200 up 25% is R250, and R250 down 25% is R187.50 (percentage calculator). Watch for "was" prices that went up in October.
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Worked example 2: four friends, one bill
- Bill R1 250, tip 10%: R125 tip, R1 375 in total (split the bill calculator).
- Split four ways: R343.75 each. Nobody carries 75 cents, so the table rounds up.
- Rounded up to R350 each: R1 400 paid, and the tip becomes R150 - 12% of the bill. Fine if the service earned it - just know you chose 12%.
- Tip on the bill before any voucher or discount, since the waiter carried the full meal. Tipping is customary in South Africa, not a legal charge - a service fee already on the bill is different, so check before you tip twice.
Worked example 3: simple interest, compound interest and growth
- Simple interest is on the original amount only: R10 000 x 9% x 10 years = R9 000, so R19 000 at the end (simple interest calculator).
- Compound interest pays interest on interest: R10 000 x 1.09^10 = R23 673.64 - R4 673.64 more for the same rate and term (compound interest calculator).
- Most savings accounts and loans compound. Simple interest turns up on some short loans, interest on arrear debts and in the school syllabus.
- Doubled your money in 10 years? That is 7.18% a year - R100 000 to R200 000 (growth rate calculator). It is not "10% a year": 100% over 10 years compounds.
- After inflation at 4.4% a year, that 7.18% is 2.66% real growth - 1.0718 / 1.044 - 1 (inflation calculator). The real number is the one that buys things.
What to do before Black Friday
- Write down the price of anything you want today, so you can tell a real discount from a raised "was" price on 27 November.
- Work out the % off from the two prices yourself - not from the banner.
- Paying with credit? The interest can wipe out the discount - check it on the credit card repayment calculator first.
Sources
- Siyavula, Mathematics Grade 10, Chapter 9 Finance and growth, 9.3 Compound interest (simple vs compound R10 000 at 9% for 10 years; worked example 5, R30 000 at 6% for 4 years): siyavula.com, read by CalcSA 16 October 2026.
- SARS, Value-Added Tax - standard rate 15% and the tax fraction 15/115: sars.gov.za.
- Stats SA, Consumer Price Index August 2026 (4.4% a year, the inflation figure used here): statssa.gov.za.
Calculators used in this post
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