There is a pair of rabbit brothers running a race in the forest. The younger brother runs 10 steps first, and then the elder brother starts to catch up. If the time for the younger brother to run 4 steps is equal to the time for the older brother to run 3 steps, and the distance for the older brother to run 5 steps is equal to the distance for the younger brother to run 7 steps, then the rabbit How many steps does it take for the elder brother to catch up with the younger brother?
森林里有一对兔子兄弟赛跑，弟弟先跑10步，然后哥哥开始追赶，若弟弟跑4步的时间等于哥哥跑3步的时间，哥哥跑5步的距离等于弟弟跑7步的距离，那么兔子哥哥跑多少步才能追上弟弟．
<gadget id="calculator">3 * 7</gadget>
<output>21</output>

<gadget id="calculator">4 * 5</gadget>
<output>20</output>

<gadget id="calculator">21 - 20</gadget>
<output>1</output>

<gadget id="calculator">10 / 1</gadget>
<output>10</output>

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

<gadget id="calculator">21 / 7</gadget>
<output>3</output>

<gadget id="calculator">50 * 3</gadget>
<output>150</output>

<result>150</result>