The storybook is one unit plus an extra $2; the eraser packet is one unit. Together they make $12.

Erasers?Storybook?$2$12

Plain numbers are given; italic underlined numbers are worked out.

  1. 2 units + $2 → $12
  2. 2 units → $12 − $2 = $10
  3. 1 unit → $10 ÷ 2 = $5

The packet of erasers costs $5.

Check: $5 (erasers) + $7 (storybook) = $12. ✓