Xiaoliang and his father took a taxi to go on an outing. The rental fee is 20 yuan within 10 kilometers, and the rental fee is 3 yuan for each kilometer exceeding 10 kilometers, and the total rental fee is 50 yuan when you get off the bus. How many kilometers did the taxi travel.
小亮和爸爸坐出租车去郊游．10千米以内租费20元，超过10千米时，超过部分每千米租费3元，下车时共交租费50元．出租车行了多少千米．
<gadget id="calculator">50 - 20</gadget>
<output>30</output>

<gadget id="calculator">30 / 3</gadget>
<output>10</output>

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

<result>20</result>