Step-by-step explanation:
Half life= 19yrs
K= 0.693/19yr
K= 0.036
In Nt = - kt + ln No
ln 9000 = - 0.036(t) + ln 63000
9.105 = - 0.036t + 11.051
-0.036t = -1.946
t = 54.06years
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