5 pencils cost 15 yuan: price of 12 pencils?

At a stationery shop, 5 pencils cost 15 yuan in total. At the same unit price, how much do 12 pencils cost?

Step-by-step

1. 15 ÷ 5 = 3
   Find the unit price first (reduce to one)
2. 3 × 12 = 36
   Unit price × quantity

Why it works

The price of one pencil is the total cost divided by the number of pencils; with the unit price unchanged, the total cost is proportional to the number of pencils.

Common mistakes

• Dividing 15 by 12 instead of by the 5 pencils bought.
• Multiplying 15 by 12 instead of first finding the 3 yuan unit price.

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