Land B is between A and C. A goes from point B to point A to deliver a letter, and 2 minutes after departure, B sets off from point B to point C to deliver another letter. Two minutes after B set off, C found that A and B had just turned the two letters upside down, so he set off from B to chase A and B by bicycle in order to exchange the letters. It is known that the speeds of A and B are equal, and the speed of C is twice the speed of A and B. How long does it take C to return to B after the letter is exchanged?
B地在A，C两地之间．甲从B地到A地去送信，出发2分钟后，乙从B地出发到C地去送另一封信．乙出发后2分钟，丙发现甲、乙刚好把两封信拿颠倒了，于是他从B地出发骑车去追赶甲、乙，以便把信换过来．已知甲、乙速度相等，丙的速度是甲、乙速度的2倍，问丙从出发到把信换过来后返回B地至少要多少时间？
<gadget id="calculator">2 - 1</gadget>
<output>1</output>

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

<gadget id="calculator">2 + 2 + 2 + 2</gadget>
<output>8</output>

<gadget id="calculator">8 / 1</gadget>
<output>8</output>

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

<gadget id="calculator">8 * 2</gadget>
<output>16</output>

<gadget id="calculator">2 + 8 + 8 + 16 + 2</gadget>
<output>36</output>

<gadget id="calculator">36 / 2 / 1</gadget>
<output>18</output>

<gadget id="calculator">36 + 18</gadget>
<output>54</output>

<result>54</result>