During an earthquake, the center of the earthquake propagates a longitudinal wave and a transverse wave in all directions at the same time. The propagation speed of the longitudinal wave is 3 km/s, and the propagation speed of the transverse wave is 2 km/s. In a certain earthquake, after the earthquake detection point receives the longitudinal wave of the earthquake with a seismograph, the shear wave of the earthquake is received after 20 seconds, so how many kilometers away is the earthquake center from the earthquake detection point?
地震时，地震中心同时向各个方向传播出纵波和横波，纵波的传播速度是3千米/秒，横波的传播速度是2千米/秒．某次地震，地震检测点用地震仪接收到地震的纵波之后，隔了20秒接收到这个地震的横波，那么这次地震的地震中心距离地震检测点多少千米．
<gadget id="calculator">1 / 2</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">1 / 3</gadget>
<output>1/3 = around 0.333333</output>

<gadget id="calculator">(1/2) - (1/3)</gadget>
<output>1/6 = around 0.166667</output>

<gadget id="calculator">20 / (1/6)</gadget>
<output>120</output>

<result>120</result>