Answer:
0.14 lb
Explanation:
The half-life of U-235 (703.8 ×10⁶ yr) is the time it takes for half the U to decay.
After one half-life, half (50 %) of the original amount will remain.
After a second half-life, half of that amount (25 %) will remain, and so on.
We can construct a table as follows:
No. of Fraction Amount
half-lives t/(yr × 10⁶) remaining remaining/lb
1 703.8 ½ 1.1
2 1408 ¼ 0.55
3 2111 ⅛ 0.28
4 2815 ¹/₁₆ 0.14
We see that 2815 × 10⁶ yr is four half-lives, and the amount of U-235 remaining is 0.14 lb.
