A stationery store buys a batch of photo albums at 4.5 yuan each and sells them at 5.4 yuan each. When there are 4 copies left, the profit is 50.4 yuan in addition to the cost. Q: How many photo albums does this store buy?
文化用品商店以每本4.5元购进一批相册，以每本5.4元售出．卖到还剩4本时，除成本外还获得利润50.4元．问：这个商店购进相册多少本？
<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>