Isotope of sodium has half life of 15 hrs Sample of isotope

Isotope of sodium has half life of 15 hrs Sample of isotope has a mass of 3 g. Find amount remaining after t hours. And use logarithm to estamite time required for mass to be reduced to 0.03g

3^t/15=c where c is the current mass First set up an equation
3^t/15=.03
t/15=log₃.03

t=15log₃.03