The fast, medium and slow cars start from A to B at the same time, and at the same time, Dongdong walks along the road to A at a speed of 100 meters per minute. It is known that 30 minutes after the departure of the express train, Dongdong is encountered on the way, and 35 minutes after the departure of the medium train, Dongdong is encountered. It took the three cars 100 minutes, 120 minutes, and 150 minutes to reach point B respectively. I would like to ask: How long does it take for the slow train to meet Dongdong?
快、中、慢三辆车同时从甲地出发开往乙地，与此同时冬冬以每分钟100米的速度沿公路走向甲地．已知快车出发30分钟后在途中遇上冬冬，中车出发35分钟后遇上冬冬．三辆车到达乙地的时候分别用了100分钟、120分钟、150分钟．请问：慢车出发多长时间后可以遇上冬冬？
<gadget id="calculator">35 - 30</gadget>
<output>5</output>

<gadget id="calculator">120 / 100</gadget>
<output>6/5 = around 1.2</output>

<gadget id="calculator">(6/5) * 30</gadget>
<output>36</output>

<gadget id="calculator">36 - 35</gadget>
<output>1</output>

<gadget id="calculator">5 / 1</gadget>
<output>5</output>

<gadget id="calculator">100 * 5</gadget>
<output>500</output>

<gadget id="calculator">500 * 36</gadget>
<output>18_000</output>

<gadget id="calculator">150 / 100</gadget>
<output>3/2 = around 1.5</output>

<gadget id="calculator">(3/2) * 30</gadget>
<output>45</output>

<gadget id="calculator">36 / 45</gadget>
<output>4/5 = around 0.8</output>

<gadget id="calculator">500 * (4/5)</gadget>
<output>400</output>

<gadget id="calculator">400 * 30</gadget>
<output>12_000</output>

<gadget id="calculator">18_000 - 12_000</gadget>
<output>6_000</output>

<gadget id="calculator">400 + 100</gadget>
<output>500</output>

<gadget id="calculator">6_000 / 500</gadget>
<output>12</output>

<gadget id="calculator">12 + 30</gadget>
<output>42</output>

<result>42</result>