Current Electricity: JEE Main 2020 Physics question

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 : _______