Nuclear medicine often uses radioactive samples as a tracer to monitor processes inside the body. Suppose a nuclear reactor makes a certain isotope sample with an initial activity of 170. mCi, and the sample gets delviered to a hospital 18.5 hours later, when its activity has become 50.2 mCi. Luckily, the sample remains usable until its activity drops below 18.3 mCi. How long after the delivery is this sample usable?