V = 240 ( 1 - e -(rt)L )
given R = 1 ohm
given L = 200mH
calculate the time when the voltage is expected to be 140v
been trying for ages just cant crack it
First you should make sure you post the correct equation. I assume it's a series RL circuit so note the missing vinculum I have added (in red).
Now substitute the given data into the equation (note that 200mH = 0.2 H):
and I'm sure you can do the finishing touches.
(Since I've worked in SI units the time you get will be measured in seconds).