Answer:
From what I can see of the problem, you CANNOT solve for the half-life of U 235 AND then solve for the length of time to determine the 20% decay.
U-235 half-life is 704,000,000 years. (Wikipedia)
The elapsed time formula = half-life * [log (Beginning Amount / ending amount) / log 2]
elapsed time = 7.04 x 10^8 * [log (100 % / 80%) / log 2]
elapsed time =7.04 x 10^8 * [log (1.25) / .30103]
elapsed time =7.04 x 10^8 * [0.096910 / .30103]
elapsed time = 7.04 x 10^8 * 0.321928047
elapsed time = 226,637,000 years
Step-by-step explanation: