The original price of the park ticket is 20 yuan. After the ticket price is reduced, the number of spectators has doubled, and the ticket revenue has increased by 50%. How much is the ticket price cheaper?
公园门票原价20元，票价优惠后，观众人数增加了1倍，门票收入上涨50%，则票价便宜了多少元．
<gadget id="calculator">50 / 100</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">1 + (1/2)</gadget>
<output>3/2 = around 1.5</output>

<gadget id="calculator">20 * (3/2)</gadget>
<output>30</output>

<gadget id="calculator">30 / 2</gadget>
<output>15</output>

<gadget id="calculator">20 - 15</gadget>
<output>5</output>

<result>5</result>