The store stipulates that if you buy no more than 10 towels at a time, each towel is 5 yuan; Xiao Ming bought some towels, and the average price was 4.5 yuan each when he got home. How many towels did Xiao Ming buy in total?
商店规定：一次购买毛巾不超过10条，每条5元；超过10条，超过的部分每条只要4元．小明买了一些毛巾，回家一算平均每条4.5元，小明共买了多少条毛巾？
<gadget id="calculator">10 * 5</gadget>
<output>50</output>

<gadget id="calculator">10 * 4</gadget>
<output>40</output>

<gadget id="calculator">50 - 40</gadget>
<output>10</output>

<gadget id="calculator">4.5 - 4</gadget>
<output>0.5</output>

<gadget id="calculator">10 / 0.5</gadget>
<output>20</output>

<result>20</result>