Answer:
2.10 × 10⁹ yr
Step-by-step explanation:
The half-life of U-235 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/g
1 700 ½ 10.0
2 1400 ¼ 5.00
3 2100 ⅛ 2.50
4 2800 ¹/₁₆ 1.25
We see that 2100 × 10⁶ yr is three half-lives, and the amount of U-235 remaining is 2.50 g.
It takes 2.10× 10⁹ yr for the U-238 to decay to 2.50 g.
