The fast train and the slow train start from the midpoint at the same time and go opposite to each other. After 4 hours, the express train arrives at B, and the slow train is 60 km away from A. The speed of the slow train is 3/4 that of the express train. Find the distance between A and B.
快慢车同时从中点出发相背而行，4小时后，快车到B地，慢车离A60km，慢车的速度是快车的(3/4)，求AB距离．
<gadget id="calculator">3 / 4</gadget>
<output>3/4 = around 0.75</output>

<gadget id="calculator">1 - (3/4)</gadget>
<output>1/4 = around 0.25</output>

<gadget id="calculator">60 / (1/4)</gadget>
<output>240</output>

<gadget id="calculator">240 * 2</gadget>
<output>480</output>

<result>480</result>