passenger car and a truck travel towards each other from places A and B at the same time, and meet at a distance of 21 kilometers from the midpoint after 2 hours. It is known that the speed ratio of the truck and the bus is 5:7, how many kilometers is the distance between A and B?
一辆客车和一辆货车同时从甲、乙两地相向而行，2小时后在距中点21千米处相遇．已知货车和客车的速度比是5：7，甲、乙两地相距多少千米？
<gadget id="calculator">7 + 5</gadget>
<output>12</output>

<gadget id="calculator">7 / 12</gadget>
<output>7/12 = around 0.583333</output>

<gadget id="calculator">1 / 2</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">(7/12) - (1/2)</gadget>
<output>1/12 = around 0.083333</output>

<gadget id="calculator">21 / (1/12)</gadget>
<output>252</output>

<result>252</result>