It takes 9 hours for the express train to go from point A to point B, and it takes 12 hours for the slow train to go from point B to point A. The two cars set off at the same time, and after 3 hours, they were still 300 kilometers apart. Question: How many kilometers are there between A and B?
快车从A地开往B地，行完全程需要9小时，慢车从B地开往A地，行完全程需要12小时．两车同时出发，3小时后还相距300千米．问：A、B两地相距多少千米？
<gadget id="calculator">1 / 9</gadget>
<output>1/9 = around 0.111111</output>

<gadget id="calculator">1 / 12</gadget>
<output>1/12 = around 0.083333</output>

<gadget id="calculator">(1/9) + (1/12)</gadget>
<output>7/36 = around 0.194444</output>

<gadget id="calculator">(7/36) * 3</gadget>
<output>7/12 = around 0.583333</output>

<gadget id="calculator">1 - (7/12)</gadget>
<output>5/12 = around 0.416667</output>

<gadget id="calculator">300 / (5/12)</gadget>
<output>720</output>

<result>720</result>