A, B, and C ride bicycles in the same direction at the same time, and their respective speeds remain constant, and B is in the middle of A and C. A catches up with B in 20 minutes, and catches up with C in another 10 minutes. How many minutes will it take for B to catch up with C?
甲、乙、丙三人同时同向骑车，各自的速度都保持不变，乙在甲、丙的正中间．甲20分钟追上乙，又过10分钟追上丙．再过多少分钟乙追上丙．
<gadget id="calculator">1 / 15</gadget>
<output>1/15 = around 0.066667</output>

<gadget id="calculator">1 / 20</gadget>
<output>1/20 = around 0.05</output>

<gadget id="calculator">(1/15) - (1/20)</gadget>
<output>1/60 = around 0.016667</output>

<gadget id="calculator">1 / (1/60)</gadget>
<output>60</output>

<gadget id="calculator">60 - 30</gadget>
<output>30</output>

<result>30</result>