The diameter of the plant stem [[$ d $]] depends on the growing time [[$ t $]] according to the equation [[$ d = k \ sqrt {t} $]], where k> 0 is the proportionality factor. The stem of the plant is [[$ 9 $]] mm thick at the age of [[$ 36 $]] days. How thick is the stem at [[$ 100 $]] days? (Source: Finnish matriculation exam, autumn 1997)



Answer: The stem is mm after [[$ 100 $]] days.