The radioactive decay of Tl206 to Pb206 has a halflife of 42

The radioactive decay of Tl-206 to Pb-206 has a half-life of 4.20 min. Starting with 2.70 x 10^22 atoms of Tl-206, calculate the number of such atoms left after 28.5 min.

Solution

k=ln(2)/T

so, ln(Ao/A)=kt

or ln(2.7*10^22/A)=(28.5/4.2)*ln(2)

or A=2.456*10^20 atoms