Cars A and B travel toward each other successively from two places 815 kilometers apart. Car A travels 52 kilometers per hour, and Car B travels 42 kilometers per hour. A travels to 416 kilometers and meets B. How many hours before B starts?
甲乙两车先后从相距815千米的两地相向而行，甲车每小时行52千米，乙每小时行42千米．甲行到416千米处与乙相遇，乙比甲早出发多少小时？
<gadget id="calculator">815 - 416</gadget>
<output>399</output>

<gadget id="calculator">399 / 42</gadget>
<output>19/2 = around 9.5</output>

<gadget id="calculator">416 / 52</gadget>
<output>8</output>

<gadget id="calculator">(19/2) - 8</gadget>
<output>3/2 = around 1.5</output>

<result>3/2 = around 1.5</result>