When a half-life passes, only half of the mass of the sample stays. So to get to 25 g from 200, let's see how much half-lives will pass.
200÷2=100
100÷2=50
50÷2=25
The answer to that is 3 half-lives, or
3 × 5730 = 17190 years
9.5m questions
12.2m answers