Problem

A solution contains a mixture of two isotopes

A

(half-life = 10 days) and

B

(half-life = 5 days). Total activity of the mixture is

10^{10}

disintegration per second at time

t = 0

. The activity reduces to 20% in 20 days. Find (a) the initial activities of

A

and

B

(b) the ratio of initial number of their nuclei.