4 notebooks cost 20 yuan: what do 7 cost?

4 identical notebooks cost 20 yuan in total. At the same unit price, how much do 7 such notebooks cost?

Step-by-step

1. 20 ÷ 4 = 5
   Find the unit price first (reduce to one)
2. 5 × 7 = 35
   Unit price × quantity

Why it works

Dividing the total cost by the number of notebooks gives the unit price, which stays the same, so the new total is that unit price times the new number of notebooks.

Common mistakes

• Multiplying 20 by 7 without working out the price of one notebook first.
• Dividing 20 by 7, using the new number of notebooks instead of the 4 bought first.

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