The half-life for the first-order conversion of A to B is 56.5 hours. How long does it take for the concentration of A to decrease to 10.0% of its original amount?

Question

Our professor has given us the answer - 188hr but I am not entirely sure how he got it.

Answer 1

k = 0.693/t_1/2
Substitute k into the below equation.

ln(No/N) = kt
No = 100
N = 10
k from above.
Solve for t in hours.