The count rate from a radioactive sample falls from $4.0\times 10^5$ per second to $1.0\times 10^6$ per second in 20 hours .what will be the count rat 100 hours after the beginning ?

The decay constant of ${}^{238}U$ is $4.9\times 10^{-18}{s}^{-1}$ (a) What is the average -life of ${}^{238}U$? (b) What is half-life of ${}^{238}U$? (c) By what factor does the activity of a ${}^{238}U$ sample decreases in $9\times 10^9$ years ?