How to do this half-life problem

A radioactive isotope has a half-life of 36 hours. I have 49 g of it, approximately how long will I need to wait until it decays into one-tenth its original mass.

Thanks to whoever can help.

(1/2)^n = 0.1 (0.1 is the decimal amount remaining)

n log 0.5 = log 0.1

n = 3.322 (this is the number of half-lives elapsed)

36 hrs times 3.322 = 119.6 hrs

(1/2)^n = 0.1 (0.1 is the decimal amount remaining)

n log 0.5 = log 0.1

n = 3.322 (this is the number of half-lives elapsed)

36 hrs times 3.322 = 119.6 hrs