Step Response of an RC Circuit (Problem)
Is there any other way to find the i?


I would personally prefer to get \$i(t)\$ like so:

$$
i(t) = \frac{V_s(t) - v(t)}{R_1} = \frac{30 - 20+10e^{-.6t}}{10}u(t) = (1+e^{-.6t})u(t)
$$

Current \$i(t)\$ is the current through \$R_1\$, which is given by Ohm's law from the voltage accross it \$V_s(t) - v(t)\$.


  If I would consider the capacitor to be an open circuit and then calculate the i by dividing 30V with 30ohms, would I be wrong?


Yes, you would be wrong. The current varies with time until the capacitor voltage settles. This method would result in a current that is constant.