Xiaohua is practicing oral arithmetic. If he does 30 questions per minute, he can finish it one minute earlier; if he does 20 questions per minute, it will take more than one minute. How many questions are there in total?
小华在练习口算．如果他每分钟做30题，可以提前一分钟完成；如果他每分钟做20题，就要超过一分钟．口算题共有多少题？
<gadget id="calculator">30 * 1</gadget>
<output>30</output>

<gadget id="calculator">20 * 1</gadget>
<output>20</output>

<gadget id="calculator">30 + 20</gadget>
<output>50</output>

<gadget id="calculator">30 - 20</gadget>
<output>10</output>

<gadget id="calculator">50 / 10</gadget>
<output>5</output>

<gadget id="calculator">5 - 1</gadget>
<output>4</output>

<gadget id="calculator">30 * 4</gadget>
<output>120</output>

<result>120</result>