You need to go and demand your money back from that college or HS; half-life of a radioactive decay is an exponential effect, it is constant with time not with mass (which would be a linear effect - like water evaporation).
The reason radiometric time dating is even possible is because it is an exponential effect; if it was linear, at some point in the future you would have zero radioactivity left. As an exponential decay you never run out with radioactivity, just an ever-decreasing amount.
