The distance between A and B is 56 kilometers. People A, B and C are moving forward at speeds of 6 kilometers, 5 kilometers and 4 kilometers per hour respectively. A and B start from city A, and C from city B at the same time, and walk towards each other. I would like to ask: How long after departure, B is exactly at the midpoint of A and C?
A、B两城相距56千米，甲、乙、丙三人分别以每小时6千米、5千米、4千米的速度前进．甲、乙两人从A城，丙从B城同时出发，相向而行．请问：出发多长时间后，乙正好在甲和丙的中点？
<gadget id="calculator">5 + 4</gadget>
<output>9</output>

<gadget id="calculator">6 - 5</gadget>
<output>1</output>

<gadget id="calculator">9 - 1</gadget>
<output>8</output>

<gadget id="calculator">56 / 8</gadget>
<output>7</output>

<result>7</result>