Question

120 grams of radioactive substance, and now you have 15 grams, how many half lives passed

Answer 1

you can use formula:

`\tt N_t=((1)/(2)^n)N_o\\\\N_t=final\:mass\\\\N_t=initial\:mass\\\\n=number\:of\:half-life`

Input the value:

`15=120((1)/(2)^n)\\\\(15)/(120)=(1)/(2)^n\\\\(1)/(8)=(1)/(2)^n\\\\(1)/(2)^3=(1)/(2)^n\Rightarrow \boxed{\bold{n=3\:half-lives}}`