posted by Billy .
A radio substance has a half-life of 10 years. What is its annual decay rate?
You are talking about a radioactive substance-- one that emits radiation-- NOT a radio.
The annual decay rate is what you multiply the number of radioactive atoms by to get the decay rate, k, in atoms per year.
The decay rate is 0.693 divided by the half life, or 0.0693 yr^-1 in this case.