In the equation above, #a# represents the number of starting unstable atoms, #0.5# represents a loss of #50%# of the current unstable atoms, #t_"1/2"# represents a half-life time (usually in seconds, days, weeks, months or years), and #t# represents the total time passed.
For example, #t/t_"1/2" = "9 days"/"3 days"# represents a 3-day half-life passing three times.