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 $\mathrm{R} \Omega$. The voltage difference across the battery of internal resistance $20 \Omega$ is zero, the value of R (in $\Omega$ ) is $\_\_\_\_$ .