The discount calculator takes the original price, a discount in percent, and two optional percentages for tax and tip, then prints five rows: the discount amount, the price after the discount, the tip, the tax, and the total. Every row is a plain multiplication, so the whole thing checks out in under a minute with the Discount Calculator or by hand.
What the five rows mean
The discount amount is the original price times the discount divided by 100, shown with a minus sign because it is money coming off the bill. The price after the discount is the original minus that amount. The tip and the tax are percentages too, and both are applied to the price after the discount, not to the original. The total adds the three together: after-discount price plus tip plus tax.
A full pass: 100 at 20 off with a 15 tip and an 8 tax
Enter 100 as the original price and 20 as the discount: the discount row reads 20.00 and the after-discount row reads 80.00. Add 15 for tip and 8 for tax. The tip is 15 percent of 80.00, which is 12.00; the tax is 8 percent of 80.00, which is 6.40. The total is 80.00 + 12.00 + 6.40 = 98.40. A Percentage Calculator reproduces each of those multiplications if you want to verify a single step.
Why the base of the tip and tax matters
If the same 15 tip and 8 tax were charged on the original 100 instead, the tip would be 15.00 and the tax 8.00, and the bill would read 80.00 + 15.00 + 8.00 = 103.00. The tool charges both on the 80.00 that remains after the discount, so the base choice saves 4.60 on this example. The saving grows with the size of the discount, and a Tip Calculator that works on the pre-discount price shows the gap even more clearly on big tickets.
Stacked discounts: 30 then 20
When two sales stack, each pass takes its percent off the current price. Start at 100, cut 30 percent and the running price is 70.00; cut 20 percent of 70.00 and it lands on 56.00. The total reduction is 44, so the combined cut is 44 percent, not the 50 you get by adding the two labels. The order of the passes does not change the product, 0.7 times 0.8 equals 0.8 times 0.7, and a Percentage Change view of 100 to 56 confirms the 44 percent drop.
Presets, negative discounts, and rounding
The preset buttons 5, 10, 15, 20, 25, 30, and 50 write that number straight into the discount field; they are shortcuts, not limits, so 33 or 62.5 still work by typing. A negative discount is a markup: -10 on 100 adds 10.00 and the after-discount row reads 110.00. Rounding happens at print time: 199.99 at 20 percent is 39.998 before rounding, shown as 40.00, while the after-discount row keeps the unrounded value and shows 159.99, so the two displayed rows do not always add back to the original. On a bigger ticket, 1,000 at 25 percent with a 15 tip and an 8 tax cuts 250.00, leaves 750.00, adds 112.50 of tip and 60.00 of tax, and totals 922.50.
Where the money you save goes
The 4.60 from the base and the 44 from stacking are not the end of the story; they are the amount that does not leave your wallet on this purchase. If you charged the ticket, run the real total, not the sticker price, through a Credit Card Payoff plan so the balance you are actually carrying is the one being retired. Sweep the recurring savings into a Savings Goal, and if the extra each month is more than pocket change, it is exactly the kind of extra payment that cuts months off a debt snowball. The habit worth keeping is the one from calculating credit card payoff: always feed the plan the number you actually paid, after every discount.