Bridget bakes 48 loaves of bread for her bakery. She sells half of them in the morning for each. In the afternoon she sells two thirds of what she has left, and because they are not fresh, she charges only half price. In the late afternoon she sells the remaining loaves at a dollar each. Each loaf costs for her to make. In dollars, what is her profit for the day?
- A)
- B)
- C)
- D)
- E)
Answer
E
Key insight
Follow the loaves in three groups, 24, 16 and 8, to get 88 dollars of sales. Then subtract what all 48 loaves cost to make, 36 dollars.
Solution
Profit means money taken in minus money spent. Work out each part separately.
Money taken in. Follow the loaves through the day.
- Morning: half of is loaves. At each, that is .
- Afternoon: loaves are left. Two thirds of is loaves. Half price is , so she takes in .
- Late afternoon: loaves are left, at each, so .
- Total taken in: .
Money spent. She made all loaves at each: .
Profit. dollars.
The answer is .
Why this works
A problem where things are sold in stages is just careful bookkeeping: at each stage, write down how many are left and what price they sell for. The one idea to remember is that profit subtracts the cost of everything that was made, not only the items sold at full price.
The trap
Answering with the sales total 88, or taking two thirds of 48 instead of two thirds of the 24 loaves left after the morning.
Concise solution
Follow the loaves through the day.
- Morning: half of is loaves at , earning .
- Afternoon: loaves remain; two thirds of them is loaves at half price, , earning .
- Late afternoon: the last loaves at each earn .
Total revenue: .
Every loaf cost to make, so the total cost is .
Profit . The answer is .
Review in the AoPS books
- Introduction to Algebra, Chapter 3: One-Variable Linear Equations (§3.3, §3.4)
- Introduction to Algebra, Chapter 6: Ratios and Percents
- Introduction to Algebra, Chapter 7: Proportion (§7.4)
- Prealgebra, Chapter 13: Data and Statistics (§13.1)
Techniques
Set up the equation/formula and compute; no special trick needed