Beibei's family drove from Beijing to Beidaihe for vacation. They originally planned to arrive in 4 hours, but encountered heavy fog when they actually set off. The car drove 20 kilometers less per hour, and finally arrived at the destination 2 hours late. So how fast did they originally plan to drive?
贝贝一家从北京开车去北戴河度假，原计划4小时到达，可实际出发的时候遇上大雾，汽车每小时少开20千米，结果晚了2小时才到达目的地．那么他们的原计划开多快？
<gadget id="calculator">4 * 20</gadget>
<output>80</output>

<gadget id="calculator">2 * 20</gadget>
<output>40</output>

<gadget id="calculator">80 + 40</gadget>
<output>120</output>

<gadget id="calculator">2 - 4</gadget>
<output>-2</output>

<gadget id="calculator">4 + (-2)</gadget>
<output>2</output>

<gadget id="calculator">120 / 2</gadget>
<output>60</output>

<result>60</result>