9514 1404 393
Answer:
45 hours
Explanation:
The fraction remaining is ...
(25 g)/(200 g) = 1/8
We recognize this value as (1/2)^3, meaning that 3 half-lives will have passed when that amount remains. (The amount remaining is multiplied by 1/2 each time a half-life passes.)
3 half-lives is 3 × 15 hours = 45 hours
It will take 45 hours for 200 g of Sodium-24 to decay to 25 g.