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