A rock contains 12.5% of its orignal K40 (half life = 704 million years). How old is the rock?

Half-life is the time needed for a radioactive material to decay 50% of its original mass. In this question, the rock K-40 left is 12.5% of its original weight. Then, the calculation to find the rock age should be:

final mass = (1/2) ^ (age/half life) * original mass

12.5% original mass = (1/2) ^ (age/704 years) * original mass

1/8 = (1/2) ^ (age/704 years)

(1/2) ^3 = (1/2) ^ (age/704 years)

(age/704 years) = 3

age = 3 * 704 years = 2112 years