The elder sister's walking speed is 75 m/min, and the younger sister's walking speed is 45 m/min. 20 minutes after the younger sister set off, the older sister set off to chase her younger sister. Q: How many minutes will it take to catch up?
姐姐步行速度是75米/分，妹妹步行速度是45米/分．在妹妹出发20分钟后，姐姐出发去追妹妹．问：多少分钟后能追上？
<gadget id="calculator">75 - 45</gadget>
<output>30</output>

<gadget id="calculator">20 / 30</gadget>
<output>2/3 = around 0.666667</output>

<gadget id="calculator">45 * (2/3)</gadget>
<output>30</output>

<result>30</result>