Xiao Ming needs to walk 20 minutes to go to school every day, and he left 4 minutes late today. If he still wants to arrive at school on the usual time, how much faster than usual.
小明每天上学需走20分钟，今天迟出发4分钟，若他仍要按平时的时间到校，速度需比平时加快多少．
<gadget id="calculator">20 - 4</gadget>
<output>16</output>

<gadget id="calculator">1 / 16</gadget>
<output>1/16 = around 0.0625</output>

<gadget id="calculator">1 / 20</gadget>
<output>1/20 = around 0.05</output>

<gadget id="calculator">(1/16) - (1/20)</gadget>
<output>1/80 = around 0.0125</output>

<gadget id="calculator">(1/80) / (1/20)</gadget>
<output>1/4 = around 0.25</output>

<result>1/4 = around 0.25</result>