Two freight trains depart from two places at the same time and meet at a distance of 50 kilometers from the midpoint. It is known that the ratio of the speed of the slow train to the express train is 7:9. How many kilometers are the distance between the two places?
两列货车从两地同时相对开出，在距中点50千米处相遇，已知慢车与快车速度的比是7：9．两地相距多少千米？
<gadget id="calculator">50 + 50</gadget>
<output>100</output>

<gadget id="calculator">7 + 9</gadget>
<output>16</output>

<gadget id="calculator">9 / 16</gadget>
<output>9/16 = around 0.5625</output>

<gadget id="calculator">7 / 16</gadget>
<output>7/16 = around 0.4375</output>

<gadget id="calculator">(9/16) - (7/16)</gadget>
<output>1/8 = around 0.125</output>

<gadget id="calculator">100 / (1/8)</gadget>
<output>800</output>

<result>800</result>