Money & Financial Maths

Australian Money Basics

• $1 = 100 cents  |  Coins: 5c, 10c, 20c, 50c, $1, $2
• Cash totals are rounded to the nearest 5c (card payments are exact)

GST (Goods and Services Tax)

In Australia, GST is 10% added to the pre-tax price.

• Price including GST = Original price × 1.1
• GST amount on a GST-inclusive price = Price ÷ 11
• Example: Item costs $80 ex-GST → GST = $8 → Total = $88

Discounts and Sale Prices

• Discount amount = Original × (discount %) ÷ 100
• Sale price = Original − Discount
• Shortcut: 25% off = pay 75% → $120 × 0.75 = $90
• Shortcut: 30% off = pay 70% → $200 × 0.70 = $140

Profit and Loss

• Profit = Selling price − Cost price
• Loss = Cost price − Selling price
• Profit % = (Profit ÷ Cost price) × 100
• Example: Buy for $40, sell for $55 → Profit = $15 → Profit % = 37.5%

Simple Interest

Interest = Principal × Rate × Time   (rate as a decimal, time in years)

Example: $500 at 4% per year for 3 years → Interest = 500 × 0.04 × 3 = $60

Key Tips

• For % increase/decrease, always calculate the percentage of the original amount
• "25% off" and "pay 75%" are equivalent — whichever is faster to calculate
• Always check whether a price is stated as including or excluding GST
• Round money to 2 decimal places (cents)