The half life of a substance is the time i takes for the radioactive component for a substance to decay or disintegrate to half of its initial size.
t = [t1/2 x In(Nt/No)] / In 2
where:
t1/2 = Half life = 4 minutes
No = Initial quantity of the sample = 90 mg
Nt = Amount of the sample left = 23 mg
t = time elapsed = ?
Hence,
t = [4 x In (23/90)] / -In 2
t = 7.8731645610906 minutes
Approximately to the nearest hundredth = 7.87 minutes
Therefore, there will be 23mg of substance remaining after 7.87 minutes.
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