: The decay constant of 
(electron capture to 
) is 1.8 × 10–4 s–1.
(a) What is the half-life?
(b) What is the average-life?
(c) How much time will it take to convert 25% of this isotope of mercury into gold?
a) Half-life 
b) Average-life 
c) We know,
where A= Activity of the substance
A0 = Initial activity
N = number of half lives
According to the problem,
Present activity = (1-0.25) A0
= 0.75A0
So,
1