Calculating Total Cost

Emma buys 3 notebooks at 4.50 dollars each and 2 pens at 1.75 dollars each. What is her total cost?

Calculate the cost of notebooks: $3 \times 4.50 = 13.50$ dollars = 13.50 dollars for notebooks

Calculate the cost of pens: $2 \times 1.75 = 3.50$ dollars = 3.50 dollars for pens

Add both amounts: $13.50 + 3.50 = 17.00$ dollars = Total: 17.00 dollars

Answer: Emma's total cost is 17.00 dollars.

Making Change

Marcus buys a sandwich for 8.75 dollars and pays with a 20 dollar bill. How much change should he receive?

Identify the operation: Change = Payment - Cost = We need to subtract

Set up the equation: Change $= 20.00 - 8.75$ = Ready to calculate

Calculate the change: $20.00 - 8.75 = 11.25$ = 11.25 dollars change

Answer: Marcus should receive 11.25 dollars in change.

Discount Problem

A jacket originally costs 80 dollars and is on sale for 25% off. What is the sale price?

Calculate the discount amount: $80 \times 0.25 = 20$ dollars = Discount is 20 dollars

Subtract discount from original price: $80 - 20 = 60$ dollars = Sale price: 60 dollars

Alternative method (multiply by remaining %): $80 \times 0.75 = 60$ dollars = Same answer: 60 dollars

Answer: The sale price of the jacket is 60 dollars.

Unit Price Comparison

A 12-pack of juice boxes costs 6.00 dollars. A 20-pack costs 9.00 dollars. Which is the better value?

Find unit price of 12-pack: $6.00 \div 12 = 0.50$ dollars per box = 0.50 dollars each

Find unit price of 20-pack: $9.00 \div 20 = 0.45$ dollars per box = 0.45 dollars each

Compare unit prices: $0.45 < 0.50$ = 20-pack is cheaper per box

Answer: The 20-pack is the better value at 0.45 dollars per box (vs 0.50 dollars per box).

Multi-Step Problem with Tax

Sofia buys a book for 15.00 dollars and a backpack for 35.00 dollars. If sales tax is 8%, what is her total cost including tax?

Find the subtotal: $15.00 + 35.00 = 50.00$ dollars = Subtotal: 50.00 dollars

Calculate the tax: $50.00 \times 0.08 = 4.00$ dollars = Tax: 4.00 dollars

Add tax to subtotal: $50.00 + 4.00 = 54.00$ dollars = Total with tax: 54.00 dollars

Answer: Sofia's total cost including tax is 54.00 dollars.

Mistake: Forgetting to multiply quantity times price

Why: When buying multiple items, students sometimes just add the price once instead of multiplying.

Correct: If you buy 4 items at 3 dollars each, the cost is $4 \times 3 = 12$ dollars, not just 3 dollars.

Mistake: Subtracting discount percentage from price instead of calculating the actual discount

Why: A 20% discount on 50 dollars is not 50 - 20 = 30 dollars.

Correct: First calculate: $50 \times 0.20 = 10$ dollars discount. Then: $50 - 10 = 40$ dollars.

Mistake: Confusing which number to divide when finding unit price

Why: To find price per item, some students divide items by price instead of price by items.

Correct: Unit price = Total price divided by number of items. For 6 dollars for 3 items: $6 \div 3 = 2$ dollars per item.

Mistake: Calculating tax on the original price instead of the sale price

Why: Tax is charged on what you actually pay. If you take the tax from the original price and add it to the sale price, the total comes out too high.

Correct: For 100 dollars with 20% off and 10% tax: sale price $100 \times 0.80 = 80$ dollars, tax $80 \times 0.10 = 8$ dollars, total $80 + 8 = 88$ dollars (not $80 + 10 = 90$ dollars).

Shopping with a Budget

When shopping with limited money, you need to track your running total to stay within budget.

You have 25 dollars. A shirt costs 12.99 dollars and socks cost 5.49 dollars. Your total is $12.99 + 5.49 = 18.48$ dollars, leaving you $25 - 18.48 = 6.52$ dollars.

Comparing Store Prices

Smart shoppers compare unit prices to find the best deal, especially when packages come in different sizes.

Store A sells 6 apples for 3.00 dollars (0.50 dollars each). Store B sells 8 apples for 3.60 dollars (0.45 dollars each). Store B has the better price per apple.

Restaurant Bills and Tips

When dining out, you need to calculate tips based on the bill amount and sometimes split costs with friends.

A meal costs 32 dollars. A 15% tip is $32 \times 0.15 = 4.80$ dollars. Total with tip: $32 + 4.80 = 36.80$ dollars.

Q: How do I convert a percentage to a decimal for calculations?

A: Divide the percentage by 100. For example, 25% becomes $25 \div 100 = 0.25$. Then multiply by this decimal.

Q: Should I calculate tax before or after a discount?

A: Apply the discount first, then calculate the tax on the discounted price: tax is charged on what you actually pay. The mistake to avoid is taking the tax from the original price and adding it to the sale price.

Q: How do I know which size is a better deal?

A: Calculate the unit price (price per item or per unit of measurement) for each size. The lower unit price is the better deal.