Use N1/N0=(1/2)^10/T and X/100 = (1/2)^24/T

Let N0 be the number of nuclei at the beginning. Number of undecayed nuclei after 33% decay = 0.67N0 Number of undecayed nuclei after 67% decay = 0.33N0 Also 0.33N0 >> 0.67N0 / 2. And in one half-life the number of undecayed nuclei becomes half.

use N/N_o=(1/2)^1/T_1/2 T_1/2=t2 - t1 t(mean)=T_1/2/0.693

let' s say C disintegrates in to A and B and initialy let's say 4units of C existed .After 1620 years 1 unit of a and 0.5of B would remain and in another 810 years A will become 0.75 and B will become 0.25 which adds up to 1 which is 1 /4 OF initial amount totally 2430 years would have passed