Dad and Xiao Ming start from the same place at the same time and run in the same direction on the circular track. Dad runs 150m per minute, and Xiao Ming runs 120m per minute. If the total length of the track is 900m, how many minutes will it take for Dad to catch up with Xiao Ming?
爸爸和小明同时从同一地点出发，沿相同的方向在环形跑道上跑步，爸爸每分钟跑150m，小明每分钟跑120m，如果跑道全长900m，问至少经过几分钟爸爸从小明身后追上小明？
<gadget id="calculator">180 - 150</gadget>
<output>30</output>

<gadget id="calculator">900 / 30</gadget>
<output>30</output>

<result>30</result>