One of the hazards of nuclear expolsion is the generation of `.^(90)Sr` and its subsequent incorportion in bones. This nuclide has a half-life of `28.1` year. Suppose one microgram was absorted by a new-born chid, how much `Sr^(90)` will remain in his bones after 20 year?

Explanation

Given `t_(1//2) = 28.1` year , `N_(0) = 10^(-6) g, t = 20` year, `N = ?`
`:' t = (2.303)/(lambda) log_(10) (N_(0))/(N)`
`:. 20 = (2.303xx28.1)/(0.693) log_(10) (10^(-6))/(N)`
`:. N = 6.1 xx 10^(-7)g`