Use the difference in the number sold to find the unit value.<br><br>
[[model:questions/solutions/q10435.png]]<br><br>
Let fiction books = \(5u\) and non-fiction books = \(4u\).<br>
&bull; Fiction sold = 40% of \(5u = 2u\).<br>
&bull; Non-fiction sold = 25% of \(4u = 1u\).<br>
&bull; Difference sold = \(2u - 1u = 1u = 60\).<br>
&bull; So \(u = 60\).<br>
&bull; Total books at first = \(5u + 4u = 9u = 9 \times 60 = 540\).<br><br>
The shop had 540 books at first.<br>
Check: Fiction = 300 (sold 120), Non-fiction = 240 (sold 60). Difference sold = 60. ✓