First of all, that's not an equation. An equation needs an 'equals' sign ( = ),
so you need something like
Amount present = a e^-0.0025t .
'a' = amount present when t = 0 .
The half-life is the time it takes for the original amount to decay to half of it.
That's just 't' when e^-0.0025t = 1/2
Take the natural log of each side: -0.0025t = ln(0.5)
Divide each side by -0.0025 : t = ln(0.5) / (-0.0025) =
(-0.69315) / (-0.0025) =
t = 277.26 minutes
( 4hrs 37min 15.5sec)
Rounded to nearest tenth: t = 277.3 minutes .