The speed ratio of the two cars A and B is 7:4. The two cars start from A and B at the same time and go towards each other. They meet at a distance of 3 kilometers from the midpoint of A and B. Find the distance between A and B. What is the distance in kilometers.
A、B两车的速度比是7：4，两车分别从甲、乙两地同时出发相向而行，在距甲、乙两地中点3千米处相遇，求甲、乙两地之间的距离是多少千米．
<gadget id="calculator">1 / 2</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">4 + 7</gadget>
<output>11</output>

<gadget id="calculator">4 / 11</gadget>
<output>4/11 = around 0.363636</output>

<gadget id="calculator">(1/2) - (4/11)</gadget>
<output>3/22 = around 0.136364</output>

<gadget id="calculator">30 / (3/22)</gadget>
<output>220</output>

<result>220</result>