There are three cars A, B, and C, each driving from A to B at a certain speed. B starts 10 minutes later than C, and catches up with C after 40 minutes. A starts 20 minutes later than B, and after 50 minutes catches up with C. Go to B, how many minutes does it take for A to catch up with C after starting off?
有甲、乙、丙三辆汽车，各以一定的速度从A地开往B地，乙比丙晚出发10分钟，经过40分钟追上丙，甲比乙晚出发20分钟，经过50分钟追上乙，那么甲出发后经过多少分钟追上丙？
<gadget id="calculator">10 + 20</gadget>
<output>30</output>

<gadget id="calculator">20 + 50</gadget>
<output>70</output>

<gadget id="calculator">70 / 50</gadget>
<output>7/5 = around 1.4</output>

<gadget id="calculator">40 + 10</gadget>
<output>50</output>

<gadget id="calculator">50 / 40</gadget>
<output>5/4 = around 1.25</output>

<gadget id="calculator">(7/5) * (5/4)</gadget>
<output>7/4 = around 1.75</output>

<gadget id="calculator">(7/4) - 1</gadget>
<output>3/4 = around 0.75</output>

<gadget id="calculator">30 / (3/4)</gadget>
<output>40</output>

<result>40</result>