Xiaoxi bought 30 exercise books A and B, paid 54 yuan, and got back 0.5 yuan. Class A exercise book is 1.5 yuan each. Type B exercise book is 2 yuan each. Ask how many exercise books of Type A you bought.
小西买甲、乙两种练习本共30本，付出54元，找回0.5元．甲种练习本每本1.5元．乙种练习本每本2元．问甲种练习本买了多少本．
<gadget id="calculator">1.5 * 30</gadget>
<output>45</output>

<gadget id="calculator">54 - 0.5 - 45</gadget>
<output>8.5</output>

<gadget id="calculator">2 - 1.5</gadget>
<output>0.5</output>

<gadget id="calculator">8.5 / 0.5</gadget>
<output>17</output>

<gadget id="calculator">30 - 17</gadget>
<output>13</output>

<result>13</result>