Work backwards: the amount left after each step is a percentage of the amount before it.<br><br>
[[model:questions/solutions/q10421.png]]<br><br>
After the bag, Kevin had $72. This is 60% of the remainder after the book (since 40% was spent on the bag).<br>
&bull; Remainder after book = $72 ÷ 60 × 100 = $72 ÷ 0.6 = $120<br>
&bull; $120 is 80% of the original amount (since 20% was spent on the book).<br>
&bull; Original amount = $120 ÷ 80 × 100 = $120 ÷ 0.8 = $150<br><br>
Kevin had $150 at first.<br>
Check: $150 − 20% = $120; $120 − 40% = $72. ✓