A young sprinter ran 90 meters with the wind in 10 seconds. Under the same wind speed, it took 10 seconds to run 70 meters against the wind. How many seconds does it take him to run 100 meters when there is no wind?
一位少年短跑选手，顺风跑90米用了10秒钟．在同样的风速下，逆风跑70米，也用了10秒钟．在无风的时候，他跑100米要用多少秒．
<gadget id="calculator">90 / 10</gadget>
<output>9</output>

<gadget id="calculator">70 / 10</gadget>
<output>7</output>

<gadget id="calculator">9 + 7</gadget>
<output>16</output>

<gadget id="calculator">16 / 2</gadget>
<output>8</output>

<gadget id="calculator">100 / 8</gadget>
<output>25/2 = around 12.5</output>

<result>25/2 = around 12.5</result>