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The half-life of a substance is the amount of time it takes for half of the substance to remain. If the initial amount of the substance is mo and the half- life is h, then the amount of the substance left after f time has past is: m(t) = mo () Note: Both and h represent a time. They must be in the same unit. For example, if 7 is in minutes but his in hours, you would need to convert to hours OR convert i to minutes so they are in the same unit. **The half-life formula is almost identical to the doubling time formula except there is a base of 1/2 (for half-life) rather than 2 (for doubling).