For why I used bc, see https://news.ycombinator.com/item?id=26293876 .

For masks, see https://gavinhoward.com/2020/08/masks-and-carbon-dioxide/ and https://gavinhoward.com/2020/08/in-science-all-factors-matte... . In essence, those who claim that masks work might be right, but they also ignore other factors that, if considered, might show that masks are worse for us overall.