The series combination of two batteries, both of the same emf 10 V, but different internal resistance of $20\,\Omega$ and $5\,\Omega$, is connected to the parallel combination of two resistors $30\,\Omega$ and $x\,\Omega$. The voltage difference across the battery of internal resistance $20\,\Omega$ is zero, the value of $x$ (in $\Omega$) is $\underline{\hspace{1cm}}$