Discount Detective

Pre-Algebra · Axiom Academy

Crack the case of the sale price: a discount peels a slice off the original, and what is left is what you pay. Your first case, Detective. The Electronics Emporium tags a pair of headphones at 80 with 25% off . The discount is the slice that comes off; the sale price is everything that remains. Press play and watch the 20 slice peel away. The discount: a slice of the original There is a shortcut hiding here. Taking off means you keep the other , so instead of subtracting you can multiply once by 0.75 : 2. The Stacked Discount Mystery Trickier case. Fashion Central takes 30% off a 100 jacket, then an extra 20% off at the register. It is tempting to add: 30\% + 20\% = 50\% off. The animation shows why that is wrong — the second discount only sees the already-reduced price, not the original. That extra is of 70 , which is only 14 — not of 100 . Add the percents and you would over-count the saving by charging the second discount against the bigger original. The same gaming console sits at three stores. The signs look different — a percent here, a dollar amount there — so you cannot trust the biggest number. A good Detective compares one thing: the final price you actually pay . Watch all three bars shrink to what you would hand over. 400 at 25\% off 400 0.75 = 300 dollars. 420 minus a flat 100 dollars. A dollar discount, not a percent. 380 at 20\% off, then 5\% off that 380 0.80 0.95 . Lowest final price wins — and it is not the store with the biggest-looking sign.

This is the written version of the interactive lesson above. See the full Pre-Algebra course.