At first, Bag : Book = 9 : 4. Let Bag = 9u, Book = 4u.<br><br>
[[model:questions/solutions/q5419.png]]<br><br>
&bull; After sale: Bag = 9u − 7, Book = 4u − 2<br>
&bull; Ratio after: (9u − 7) : (4u − 2) = 2 : 1<br>
&bull; 9u − 7 = 2(4u − 2)<br>
&bull; 9u − 7 = 8u − 4<br>
&bull; u = 3<br>
&bull; Bag at first = 9 × 3 = $27<br>
<br>
The bag cost $27 at first.<br>
Check: Book $12. After sale: Bag $20, Book $10. 20 : 10 = 2 : 1. ✓