Answer: 14.14 billion years (which is close to the age of the universe)
Explanation:half life (t1/2) U-235 = 703.8*10^6
original amountN0 at time t Nt = NO/4
t= (t1/2)*(ln(Nt/N0)/-ln2
= (703.8*10^6)*(ln0.25)/-ln2
= (703.8*10^6)*(-1.386)/-0.69
= 14.14*10^9 years
