stationery store buys several photo albums at 4.5 yuan each, and sells them for 5.4 yuan. When there are 4 copies left, the profit is 50.4 yuan in addition to the cost. How many photo albums does the store buy?
文化用品商店以每本4.5元买进相册若干本，售价为5.4元，卖到还剩4本时，除成本外还获得利润50.4元，这个商店买进相册多少本？
<gadget id="calculator">5.4 * 4</gadget>
<output>21.6</output>

<gadget id="calculator">50.4 + 21.6</gadget>
<output>72</output>

<gadget id="calculator">5.4 - 4.5</gadget>
<output>0.9</output>

<gadget id="calculator">72 / 0.9</gadget>
<output>80</output>

<result>80</result>