A 3 km long water pipeline has been laid for 5 days, and x meters are laid every day. First use the formula containing letters to indicate the number of meters that have not been paved on this day, and then calculate how many meters are left unpaved when x=400.
铺设一条长3千米的自来水管道，已经铺了5天，每天铺x米．先用含有字母的式子表示这天还没有铺的米数，再计算当x=400时，还剩多少米没有铺．
<gadget id="calculator">5 * 400</gadget>
<output>2_000</output>

<gadget id="calculator">3_000 - 2_000</gadget>
<output>1_000</output>

<result>1_000</result>