His money is 4 equal units. Three of them ($12) were spent on the book.<br><br>[[model:questions/solutions/q3406.png]]<br><br>• 3 units → $12<br>• 1 unit → $12 ÷ 3 = $4<br>• At first = 4 units → 4 × $4 = $16<br><br>He had $16 at first.<br>Check: $$\frac{3}{4}$$ of $16 = $12. ✓