Answer
Internal resistance of the battery is $10Ω$ and emf of the battery is $60 V$
Work Step by Step
When the resistance across the battery is 0 the current is generated only by the internal resistance
According to the graph when resistance across cell is 0 the current is 6A and when a 20Ω resistance across the cell, the current is 2A
$V=IR$
Emf of the cell is constant. Considering $r$ is the internal resistance of the battery,
$6A\times r=2A\times (r + 20Ω)$
$r = 10Ω$
Considering internal resistance of the cell is 10Ω we can find the emf of the battery as,
$V = 6A\times 10Ω$
$V = 60V$
