Math
posted by Billy .
A radio substance has a halflife 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.
See http://hyperphysics.phyastr.gsu.edu/hbase/nuclear/halfli2.html
The decay rate is 0.693 divided by the half life, or 0.0693 yr^1 in this case.