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):
$\displaystyle 140 = 240 \left( 1 - e^{-t/0.2}\right) = 240 \left( 1 - e^{-5t}\right)$
$\displaystyle \Rightarrow \frac{140}{240} = 1 - e^{-5 t}$
$\displaystyle \Rightarrow \frac{7}{12} - 1 = - e^{-5 t}$
$\displaystyle \Rightarrow \frac{5}{12} = e^{-5 t}$
$\displaystyle \Rightarrow \ln \frac{5}{12} = -5 t$
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).