A large truck and a small car start from the same point and drive along the same road. The large truck travels for 3 hours first, and the small car catches up with the large truck 4 hours after departure; To catch up with the big truck, how many kilometers does the big truck travel per hour?
大货车和小轿车从同一点出发沿同一条公路行驶，大货车先走3小时，小轿车出发后4小时追上大货车；如果小轿车每小时少行6千米，则出发后5小时才能追上大货车，大货车每小时行多少千米？
<gadget id="calculator">6 * 5</gadget>
<output>30</output>

<gadget id="calculator">4 / 7</gadget>
<output>4/7 = around 0.571429</output>

<gadget id="calculator">(4/7) * 3</gadget>
<output>12/7 = around 1.714286</output>

<gadget id="calculator">(4/7) * 5</gadget>
<output>20/7 = around 2.857143</output>

<gadget id="calculator">5 - (12/7) - (20/7)</gadget>
<output>3/7 = around 0.428571</output>

<gadget id="calculator">30 / (3/7)</gadget>
<output>70</output>

<gadget id="calculator">70 * (4/7)</gadget>
<output>40</output>

<result>40</result>