The discounted price is known: find the original without adding the same percentage
By Erik Nilsen · Kroni3 min read
If you know the price after a percentage discount, adding the same percentage does not recover the original price. The discount was calculated from the larger original amount. Use the share remaining, then check your answer by applying the discount forwards.
Find the percentage your child pays
In a hypothetical calculation, an optional item costs 80 units after a 20 per cent discount. You want to find the original price matching those numbers. These arbitrary prices are a maths exercise, rather than evidence of any shop's actual earlier price.
A 20 per cent discount leaves 100 − 20 = 80 per cent of the original price. Therefore, 80 units represent 80 per cent. One per cent is 80 / 80 = 1 unit. The whole original price, 100 per cent, is 100 units.
You can also write 80 / 0.80 = 100. Check by calculating 20 per cent of 100, a discount of 20 units. Then 100 − 20 = 80, the discounted price you started with.
Adding 20 per cent to 80 instead gives 80 + 16 = 96. But 20 per cent of 96 is 19.20, and 96 − 19.20 = 76.80. That does not return to 80. The short check shows why the method fails.
Draw the parts still remaining
Draw ten equal boxes representing the original price. Cross out two, leaving eight. Divide the 80 units between those eight boxes. Each represents 10 units, so ten boxes represent 100. Your child does not need to memorise a formula if they can show what the parts mean.
A realistic snag is an actual checkout price including delivery or another extra charge. The whole total is then not 80 per cent of the product price. Separate extras from the product before working backwards and read what the offer actually covers.
Even a correct answer of 100 does not prove a shop previously sold the product for 100. It only identifies the original price fitting the stated mathematical rule. Check actual price information separately if your family needs it.
Ask your child: “What share of the original price remains, and do we recover the discounted price when we test our answer?”
Carry the calculation into your family agreement
Kroni can keep track of your child's virtual balance. The app does not transfer money, so any actual purchase is paid for outside it. Agree on what you will record, then check the amount with your child afterwards. Do not deduct a possible cost as though the purchase had already happened. Read about allowance in Kroni.
