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 R $\Omega$. The voltage difference across the battery of internal resistance 20 $\Omega$ is zero, the value of R (in $\Omega$) is : $\underline{\hspace{2cm}}$