— Shopping Math —

Practice calculating sales and GST in your head — fast. Alberta GST is assumed to be 5%.

(Sales) Quick Review

Example: A $120 jacket at 20% off. First find 10%, then double it to get 20%, subtract from the original price.

10% of 120 = 120 ÷ 10 = 12

20% of 120 = 2 × 12 = 24

Discounted price = 120 − 24 = $96

Sale price (no GST yet)

Type your answer to self‑check.

(Goods and Services Tax) Quick Review

10% of $88 = $8.80 → 5% of $88 = $4.40. Final price = price + GST.

Find 5% GST (on listed price)

Final Sale Price (discount then +5% GST)

Challenge

I) If you add GST then apply the discount, do you get the same result?

Show reasoning

Let original price be P, discount rate d, GST rate g. Discount‑then‑GST gives P × (1 − d) × (1 + g). GST‑then‑discount gives P × (1 + g) × (1 − d). Multiplication is commutative, so the two are equal.