Both father and son work in a certain factory. They walk from home to the factory. The father takes 40 minutes and the son takes 30 minutes. If the father leaves home 5 minutes earlier than the son, how many minutes will it take for the son to catch up with the father?
父亲和儿子都在某厂工作，他们从家里出发步行到工厂，父亲用40分钟，儿子用30分钟．如果父亲比儿子早5分钟离家，问儿子用多少分钟可赶上父亲？
<gadget id="calculator">1 / 40</gadget>
<output>1/40 = around 0.025</output>

<gadget id="calculator">(1/40) * 5</gadget>
<output>1/8 = around 0.125</output>

<gadget id="calculator">1 / 30</gadget>
<output>1/30 = around 0.033333</output>

<gadget id="calculator">(1/30) - (1/40)</gadget>
<output>1/120 = around 0.008333</output>

<gadget id="calculator">(1/8) / (1/120)</gadget>
<output>15</output>

<result>15</result>