Land B is between A and C. A goes from point B to point A to deliver a letter. After 6 minutes of departure, B sets off from point B to point C to deliver another letter. Twelve minutes after B set off, C at B found that A and B had taken the two letters by mistake, so he set off from B to catch up with A and B by bicycle in order to exchange the letters. It is known that the speed of C is three times that of A and twice that of B. During the whole process of exchanging letters, A and B have been moving forward, and neither of them has reached the end point. How much time?
B地在A、C两地之间．甲从B地到A地去送信，出发6分钟后，乙从B地出发到C地去送另一封信．乙出发12分钟后，在B处的丙发现甲、乙把两封信拿错了，于是他从B地出发骑车去追赶甲和乙，以便把信换过来．已知丙的速度是甲的3倍，是乙的2倍，整个换信的过程中，甲、乙两人一直在前进，而且都没有到达终点，那么丙从出发到返回B地至少要用多少时间？
<gadget id="calculator">2 - 1</gadget>
<output>1</output>

<gadget id="calculator">12 / 1</gadget>
<output>12</output>

<gadget id="calculator">12 * 2</gadget>
<output>24</output>

<gadget id="calculator">18 + 24</gadget>
<output>42</output>

<gadget id="calculator">3 - 1</gadget>
<output>2</output>

<gadget id="calculator">42 / 2</gadget>
<output>21</output>

<gadget id="calculator">12 + 12 + 21 + 21</gadget>
<output>66</output>

<result>66</result>