The series combination of two batteries, both of the same emf 10 V, but different internal resistance of 20 Ω and 5 Ω, is connected to the parallel…

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