题目

BEIJING，Jan.12(Xinhuanet)—Thick fog and haze shrouded central and northern parts of China on Friday，with Hebei and Henan provinces among the most polluted areas，monitoring data showed. An air quality report from the Ministry of Environmental Protection ranked Handan，Baoding and Shijiazhuang in Hebei and Zhengzhou in Henan as the top four most－polluted cities among 120 monitored nationwide.The Air Pollution Index in the four cities reached 500，the maximum of the index，on Friday.An API reading below 50 indicates excellent air quality，50 to 100 indicates healthy air，and 5．一种正方体硬纸盒，棱长6分米．做20个这样的硬纸盒，至少要用纸板多少平方分米？