First, a set of $n$ equal resistors of $10\Omega$ each are connected in series to a battery of E.M.F. 20 V and internal resistance $10\Omega$. A current $I$ is observed to flow. Then, the $n$ resistors are connected in parallel to the same battery. It is observed that the current is increased 20 times, then the value of $n$ is $\_\_\_\_$.