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What does half life mean? It means after the half life has passed, one half of the substance is gone. Mathematically this means . Now where did this come from? Look at (t/h), where t is time and h is the half life. When t=h that means that the the time has reached the half life and this fraction will also be 1. If the fraction is 1, then you get , which is just one half of the starting amount. This makes sense because that's what the half life definition is.

So plug in the half life time your problem gives you in that equation for h and you can now know the amount left at any time t.