On Sunday, Naughty and Xiaoxiao went to the playground to play. They like roller coaster and rafting very much. The fare of rafting is 2.5 times of that of roller coaster. They calculated that if they went on the roller coaster, they would still have 10 yuan left; if they went rafting, they would still have 20 yuan left. Naughty and Xiaoxiao can only ride the roller coaster, how much money did they bring together?
星期天，淘气和笑笑到游乐场游玩．他们很喜欢过山车和漂流二种游戏，漂流的票价是过山车票价的2.5倍．他们算了一下，如果玩过山车，所带的钱还可剩下10元；如果玩漂流，所带的钱还差20元．淘气和笑笑只能玩过山车，共带了多少元．
<gadget id="calculator">20 + 10</gadget>
<output>30</output>

<gadget id="calculator">2.5 - 1</gadget>
<output>1.5</output>

<gadget id="calculator">30 / 2 / 1.5</gadget>
<output>10</output>

<gadget id="calculator">10 * 2</gadget>
<output>20</output>

<result>30</result>