It has been 4 days to build a road with a length of 172Km, with an average of 18km per day. The rest requires 5 days to complete, how many kilometers will be repaired on average per day? (List the solution of the equation first, and then use the arithmetic method to solve it)
修一条长172Km的公路，已经修了4天，平均每天修18km．余下的要求5天修完，平均每天要修多少千米？(先列方程解答，再用算术方法解答)
<gadget id="calculator">172 - 72</gadget>
<output>100</output>

<gadget id="calculator">100 / 5</gadget>
<output>20</output>

<result>20</result>