The express train and the slow train leave from two cities that are 450 kilometers apart at the same time. After 4.5 hours, the distance between the two cars is still 90 kilometers. The speed ratio of the express train and the slow train is 9:7. How many kilometers does the slow train travel per hour?
快车和慢车同时从相距450千米的两城相对开出，4.5小时后两车还相距90千米，快车和慢车的速度比为9：7，慢车每小时行多少千米？
<gadget id="calculator">450 - 90</gadget>
<output>360</output>

<gadget id="calculator">360 / 4.5</gadget>
<output>80</output>

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

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

<gadget id="calculator">80 * (7/16)</gadget>
<output>35</output>

<result>35</result>