# MCQ 1 - A radioactive substance disintegrates by 75% in 60 minutes. What is its half-life (T1/2) ?

# MCQ 1 - A radioactive substance disintegrates by 75% in 60 minutes. What is its half-life (T1/2) ?

A. 20 minutes

B. 30 minutes

C. 45 minutes

D. 120 minutes

Answer: B. 30 minutes

We have, Amount of substance left after 'n' half lives,  (N) =〖(1/2)〗^n * N_0, Where  N_0 is initial amount of substance. 

So, 25/100 = 〖(1/2)〗^n 

or, 1/4 = 〖(1/2)〗^n

i.e. n=2

So, 2 half lives equals 60 minutes i.e. one half life is equal to 30 minutes.