Tickets for the football game are 15 yuan per ticket. After the price reduction, the audience doubles and the income increases by one-fifth. Q: How much is the price reduction for a ticket?
足球赛门票15元一张，降价后观众增加一倍，收入增加五分之一．问：一张门票降价多少元？
<gadget id="calculator">1 / 5</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">1 + (1/5)</gadget>
<output>6/5 = around 1.2</output>

<gadget id="calculator">15 * (6/5)</gadget>
<output>18</output>

<gadget id="calculator">18 / 2</gadget>
<output>9</output>

<gadget id="calculator">15 - 9</gadget>
<output>6</output>

<result>6</result>