Shopping Calculator

Pre-Algebra · Axiom Academy

You're the cashier at MathMart. Ring up a basket, add the sales tax, find the total — and figure a tip — one clean step at a time. 1. The Subtotal Is Just a Running Sum Today's basket: 2 notebooks at 2.50 , 4 pens at 1.25 , and 1 snack at 3.00 . Each item's price is a bar; stack them and the heights add up. The combined height is the subtotal — what the items cost before any tax. Multiply price by quantity for each item, then add 2. Sales Tax Is a Slice of the Subtotal MathMart's sales tax is 8% . "8%" means 8 out of every 100, or the decimal 0.08 . To find the tax, take that slice of the subtotal: 13.00 split into a hundred equal parts, then 8 of those parts shaded in. 8% means 8 per 100, so divide by 100: . 13.00 = \ 1.04 . That is the tax. 10% of 13.00 is 1.30 , and 1% is 0.13 . So 8\% = 10\% - 2 1\% = \ 0.26 = \ 1.04 . It matches. The tax is always a percent of the items' price, never of the percent itself. "8% of 13.00 " is a multiplication: turn the percent into the decimal 0.08 and multiply. The grid makes it concrete — 8 shaded columns out of 100, each column worth 0.13 . 3. The Total Is Subtotal Plus Tax The last move is the easiest: set the tax on top of the subtotal. The 13.00 bar grows by the 1.04 tax bar, and the new height is the total due — the number the customer actually pays the store. Stack, don't restart: the tax is added on top of the subtotal you already found — you don't recompute the items.

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