Math logarithm word solving problem

Question

The mass of a radioactive sample is given by M(t)= M°10^ -k•t, where t is the time in years, M° is the initial mass, and k is a constant. if 400 grams of this material decays to 40 grams I. 10 years, what is the value of k?

Answer 1

40 = 400(10^-10k)

1/10 = 10^-10k

1/10 = 1/10^10k

10 = 10^10k

1 = 10k

k = 0.1