A certain class went on a trip and bought a total of 99 tickets, which cost a total of 280 yuan. Among them, each one-way ticket is 2 yuan, and each round-trip ticket is 4 yuan. Q: How much is the difference between a one-way ticket and a round-trip ticket?
某班外出旅游，共买了99张车票，一共花了280元．其中，单程票每张2元，往返票每张4元．问，单程票与往返票相差多少张？
<gadget id="calculator">99 * 4</gadget>
<output>396</output>

<gadget id="calculator">396 - 280</gadget>
<output>116</output>

<gadget id="calculator">4 - 2</gadget>
<output>2</output>

<gadget id="calculator">116 / 2</gadget>
<output>58</output>

<gadget id="calculator">99 - 58</gadget>
<output>41</output>

<gadget id="calculator">58 - 41</gadget>
<output>17</output>

<result>17</result>