A radioactive substance has a half-life of 3 minutes. After 9 minutes, the count rate was observed to be 200. What was the count rate at zero time?

Answer 1

You can do it manually by increasing it by two for 3 times since you've gone through 3 half lives. Or you can follow the formula.
k = 0.693/t_1/2
Then ln(No/N) = kt
No = ?
N = 200
k from above
t = 9 min
Solve for No. Post your work if you get stuck.