Magazine = 1 unit, book = 3 units, dictionary = 1 unit + $7. The $24 average and $7 are given; the $72 total is worked out (3 × the average).<br><br>[[model:questions/solutions/q4699.png]]<br><br>• Total → 3 × $24 = $72<br>• 1 + 3 + 1 = 5 units, plus $7 → $72<br>• 5 units → $72 − $7 = $65<br>• 1 unit → $65 ÷ 5 = $13<br>• Book = 3 units = $39; Dictionary = $13 + $7 = $20<br>• Difference → $39 − $20 = $19<br><br>The difference between the book and the dictionary is $19.<br>Check: $39 (book) + $13 (mag) + $20 (dict) = $72; $72 ÷ 3 = $24. ✓<br><br>Alternative method (algebra):<br>• Let the magazine cost $x. Book = 3x, dictionary = x + 7.<br>• x + 3x + (x + 7) = 72  ⇒  5x + 7 = 72  ⇒  5x = 65  ⇒  x = 13.<br>• Book = 39, dictionary = 20, difference = 39 − 20 = $19.