you can use formula:
[tex]\tt N_t=(\dfrac{1}{2}^n)N_o\\\\N_t=final\:mass\\\\N_t=initial\:mass\\\\n=number\:of\:half-life[/tex]
Input the value:
[tex]15=120(\dfrac{1}{2}^n)\\\\\dfrac{15}{120}=\dfrac{1}{2}^n\\\\\dfrac{1}{8}=\dfrac{1}{2}^n\\\\\dfrac{1}{2}^3=\dfrac{1}{2}^n\Rightarrow \boxed{\bold{n=3\:half-lives}} [/tex]
