What is the half-life of an isotope that decays to 6.25% of its original activity in 18.9 hours?

Radioactive material obeys 1st order decay kinetics,
For 1st order reaction, we have 
k = 
where, k = rate constant of reaction

Given: Initial conc. 100, Final conc. = 6.25, t = 18.9 hours

∴ k =  = 0.1467 hours^(-1)

Now, for 1st order reactions: half life =  = 4.723 hours.

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