The original price of a concert ticket is 80 yuan per piece, but it will be reduced to 60 yuan per piece after the performance starts. A ticket window sold a total of 180 tickets of this kind, with an income of 13,800 yuan. How many tickets were sold after the show started?
一种演唱会门票原价80元/张，演出开始后降到60元/张．一个售票窗口共卖出这种门票180张，收入13800元．演出开始后卖出门票多少张？
<gadget id="calculator">80 * 180</gadget>
<output>14_400</output>

<gadget id="calculator">14_400 - 13_800</gadget>
<output>600</output>

<gadget id="calculator">80 - 60</gadget>
<output>20</output>

<gadget id="calculator">600 / 20</gadget>
<output>30</output>

<result>30</result>