The distance between A and B is 10 kilometers. After A starts from A and walks for 45 minutes, B also starts from A by bicycle. Finally, A and B arrive at B at the same time. A begins to walk at a speed of 104 meters per minute, and then slows down to 84 meters per minute at a certain point on the way. B rides a bicycle at a speed of 210 meters per minute, but rests for 5 minutes on the way. Ask: How long does it take for A to start decelerating after starting? ?
A、B两地相距10千米，甲从A地出发步行45分钟后，乙骑自行车也从A地出发，最后甲乙两人同时到达B地．甲开始步行的速度为每分钟104米，途中某时刻后减速为每分钟84米，乙骑自行车的速度始终是每分钟210米，但途中休息5分钟，问：甲出发后多少时间开始减速的？
<gadget id="calculator">10 * 1_000</gadget>
<output>10_000</output>

<gadget id="calculator">1_000 / 21</gadget>
<output>1_000/21 = around 47.619048</output>

<gadget id="calculator">84 * (1_000/21)</gadget>
<output>4_000</output>

<gadget id="calculator">84 * 50</gadget>
<output>4_200</output>

<gadget id="calculator">10_000 - 4_000 - 4_200</gadget>
<output>1_800</output>

<gadget id="calculator">104 - 84</gadget>
<output>20</output>

<gadget id="calculator">1_800 / 20</gadget>
<output>90</output>

<result>90</result>