Question

radioisotope actinium-225 has a half-life of 10 days. if i began with 16kg of this isotope how much will remain after 40 days

Answer 1

\(\large Q = Q_0\left(\frac 12\right)^{t/\tau}\)
where \(\large Q\) is the quantity after time \(\large t\),
\(\large Q_0\) is the starting quantity
\(\large \tau \) is the half-life.

Plug the numbers in and find Q when t = 40 days.

Answer 2

\(\large Q_0 = 16~\text{kg.}; ~~~ \tau = 10~\text{days.}; ~~~ t = 40 \text{ days.}\)

Plug them into the formula on line 1 in the previous reply and find Q.