If two cars A and B start from A and B at the same time and drive towards each other, they can meet each other on the way within 4 hours. It is known that the speed ratio of cars A and B is 3:4. According to this speed, if the two cars want to meet at the midpoint of A and B, how many hours should car A leave in advance?
甲、乙两车如果从A、B两地同时开出，相向而行，4小时能在途中相遇．已知甲、乙两车的速度比是3：4，照这样的速度，如果两车要在A、B两地的中点相遇，甲车应提前多少小时开出．
<gadget id="calculator">3 + 4</gadget>
<output>7</output>

<gadget id="calculator">4 / 2</gadget>
<output>2</output>

<gadget id="calculator">7 * 2</gadget>
<output>14</output>

<gadget id="calculator">14 / 3</gadget>
<output>14/3 = around 4.666667</output>

<gadget id="calculator">14 / 4</gadget>
<output>7/2 = around 3.5</output>

<gadget id="calculator">(14/3) - (7/2)</gadget>
<output>7/6 = around 1.166667</output>

<result>7/6 = around 1.166667</result>