Three people, A, B, and C, A travels towards each other from point A, and B and C travel toward each other from point B. A and B meet at point C on the way. The speed of A is 2.5 times that of A, and the meeting point is 15 kilometers away from C. Find the distance between A and B.
甲乙丙三人，甲从A，乙丙从B点相向而行，甲和乙在途中C点相遇，此时甲转身向A点行驶，丙到A点时，正好追上甲，已知乙的速度是甲的2.5倍，相遇点距丙15千米，求AB相距多少．
<gadget id="calculator">2 + 5</gadget>
<output>7</output>

<gadget id="calculator">2 / 7</gadget>
<output>2/7 = around 0.285714</output>

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

<gadget id="calculator">(2/7) * 2</gadget>
<output>4/7 = around 0.571429</output>

<gadget id="calculator">1 / (4/7)</gadget>
<output>7/4 = around 1.75</output>

<gadget id="calculator">(2/7) * (7/4)</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">1 - (1/2) - (2/7)</gadget>
<output>3/14 = around 0.214286</output>

<gadget id="calculator">15 / (3/14)</gadget>
<output>70</output>

<result>70</result>