Question 930855
vc = v(1-e^(-t/CR)) 
vc = v -ve^(-t/CR)
ve^(-t/CR) = v - vc
e^(-t/CR) = (v - vc) / v
e^(-t/CR) = 1-c
use logarithm rule
-t/CR = ln (1-c)
t = -CR * ln(1-c)