Radium - $221$ Has a Half-Life Of $30 \mathrm {~s}$ Suppose We Have A Half-Life

Essay

Radium - $221$ has a half-life of $30 \mathrm {~s}$ . Suppose we have a $300 \mathrm {~g}$ sample.
(a) Find a formula for the mass remaining after $t$ seconds
b) How much of the sample remains after $200$ seconds?
c) After how long will only $20 \mathrm {~g}$ remain?