Week 2 of 5

Percentages in the wild: discounts, GST, unit prices and ratio

Session 1 · SOLVE: Link Ideas

Is it really a deal?

10 minDiscount maths: 35% off $180 means paying 65% of $180 = 0.65 × $180 = $117. Two ways, one answer.
15 minSale sleuth: work out the real discount on catalogue items ('Was $80, now $59.99': what percentage off?).
15 minWhich is better: 30% off, 'buy two, get one free' or '$20 off when you spend $100'? It depends. Test each with real prices.
10 minGST: most prices in New Zealand include 15% GST. If a price before GST is $40, what is the price with GST? Why is 'plus GST' on a sign worth noticing?

Session 2 · SOLVE: Link Ideas

10 minUnit price: $4.50 for 500 g and $7.80 for 1 kg. Which is cheaper per 100 g? Show your working.
20 minCheck five real products in two sizes (from a catalogue or online). Is the bigger pack always cheaper per unit?
15 minRatio: a recipe makes 12 muffins with 3 cups of flour to 1 cup of sugar. How much of each for 36 muffins? For 30? Share market-day costs between two classes in the ratio 2 : 3.
5 minTohu asks one question about a deal you think is good.

Getting started

Work out the discount first (35% of $180 = $63), then subtract it. Later, try the faster 65% method.

Stretch

A price goes up 50% and then comes down 50%. Is it back where it started? Explain with numbers.

Checkpoint

Correct discounts and unit prices, with a clear reason for which deal is best for a given shopper.

Activities for this week

Sale sleuth: Which discount really saves the most?
Unit price showdown: Is the bigger pack always the better buy?
Market-day stall: cost, price and profit: What price covers our costs, sells well and makes a fair profit?

Useful this week