# http://news.bbc.co.uk/2/hi/programmes/more_or_less/8735812.stm
one_child <- expand.grid(
	sex=c("Boy", "Girl"), wday=c("Mo", "Tue", "We", "Th", "Fr", "Sa", "Su")
) |> 
transform(sex_wday=paste(sex, "-", wday)) |>
subset(select="sex_wday") |>
unlist() |>
unname()

two_children <- expand.grid(first=one_child, second=one_child, stringsAsFactors=FALSE) |>
transform(
	tuesday_boy=grepl("Boy - Tue", first) | grepl("Boy - Tue", second),
	two_boys=grepl("Boy", first) & grepl("Boy", second)
)

sum(two_children$tuesday_boy & two_children$two_boys) /
sum(two_children$tuesday_boy)