The younger brother started from home and walked to the park at a speed of 4 kilometers per hour. Two hours later, the elder brother left home and rode a bicycle at a speed of 20 kilometers per hour to catch up with his younger brother. Ask how long it will take for the elder brother to catch up with the younger brother?
弟弟以每小时4千米的速度从家里出发步行去公园，2小时后，哥哥离开家以每小时20千米的速度骑自行车去追赶弟弟．问哥哥多长时间后能追上弟弟？
<gadget id="calculator">20 - 4</gadget>
<output>16</output>

<gadget id="calculator">2 / 16</gadget>
<output>1/8 = around 0.125</output>

<gadget id="calculator">4 * (1/8)</gadget>
<output>1/2 = around 0.5</output>

<result>1/2 = around 0.5</result>