A sixth-grade student travels to a certain place by car and buys 99 tickets, which cost a total of 28 yuan. Among them, each one-way ticket is 0.2 yuan, and the round-trip ticket is 0.4 yuan. How much is the difference between a one-way ticket and a round-trip ticket.
六年级同学乘汽车到某地旅游，买车票99张，共花28元．其中单程票每张0.2元，往返票0.4元，单程票和往返票相差多少张．
<gadget id="calculator">39.6 - 28</gadget>
<output>11.6</output>

<gadget id="calculator">0.4 - 0.2</gadget>
<output>0.2</output>

<gadget id="calculator">11.6 / 0.2</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>