and 
Initially,
10% of the decays come from 
How long one must
wait until 90% do so ?
Answer:
and 
be the initial
activities of 
and 
respectively
and 
and 
be their
activities at any instant t.
Then, as required by
the question, initially
[
of
total activity]
or
?(1)
Also, at a
time t
[
of
total activity at time t ]
or
or
Dividing (2)
by (1), 
or
But
or
Taking
natural log of both sides,
or
(3)
Now
and
From
eqn. (3),
or
days.
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