The distance between A and B is 56 km. There are three people A, B, and C. A and B depart from city A, and C departs from city B at the same time. Walk towards each other. A, B, and C are traveling at speeds of 6 km, 5 km, and 4 km/h, respectively. Find how many hours after departure, B is at the midpoint between A and C.
A、B两城相距56千米．有甲、乙、丙三人．甲、乙从A城，丙从B城同时出发．相向而行．甲、乙、丙分别以每小时6千米、5千米、4千米的速度行进．求出发后经多少小时，乙在甲丙之间的中点处．
<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>