Given :
The half life of uranium 232 is 70 years.
To Find :
How many half lives will it take for 10 grams of it to be reduced to 1.25 grams.
Solution :
We know, formula of radioactive decay is :
[tex]\dfrac{N}{N_o}=(\dfrac{1}{2})^n[/tex]
Here, [tex]N_o[/tex] is initial amount and N is remaining amount.
Putting all given values in above equation, we get :
[tex](\dfrac{1}{2})^n = \dfrac{1.25}{10}\\\\(\dfrac{1}{2})^n = \dfrac{1}{8}\\\\n = 3[/tex]
Therefore, it takes 3 half lives i.e. 210 years to reduced to 1.25 grams.
