HELP! Precalculus Radioactive Isotope problem?

If a radioactive isotope decays according to the formula Q(t)= Q0*2^(-1/2), where t is measured in years, find the portion of the initial amount remaining after a. 1 year, b. 12 years, c. 100 years. The answers are a. .94Q0, b. .5Q0, c. .003Q0. I have tried for hours to figure out this problem, but cannot. I am totally missing someting. Please Help ASAP....