Khimraj
Last Activity: 7 Years ago
Let the two particle are a and b. and as decay constant

are given by
a = ln2/(t
1/2)
a = ln2/1620
b = ln2/(t
1/2)
b = ln2/810
Total decay constant

=
a+
b
= ln2/1620 + ln2/810 = 3*ln2/1620
when (1/4)th of metal remains
N0/4 = N0e-λt
λt = ln4
t = 2ln2/λ = 2*ln2/λ = (2/3)*1620 = 1080 years.
Hope it clears. If you like answer then please approve it.