Cars A and B leave opposite sides at the same time and meet at a distance of 21 kilometers from the midpoint. Given that the speed ratio of cars A and B is 7:5, what is the distance in kilometers between the two places?
甲、乙两车同时从两地相对开出，在离中点21千米处相遇．已知甲乙两车的速度比是7：5，则两地相距多少千米．
<gadget id="calculator">7 + 5</gadget>
<output>12</output>

<gadget id="calculator">7 - 5</gadget>
<output>2</output>

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

<gadget id="calculator">21 * 1</gadget>
<output>21</output>

<gadget id="calculator">12 * 21</gadget>
<output>252</output>

<result>252</result>