Answer
The internal resistance of the cell is approximately $1.2Ω$
Work Step by Step
Current across the resistor is,
$V=IR$
$8.5V=I×20Ω$
$I=0.425A$
Same current applies across the internal resistance
Potential drop across the internal resistance can be taken as,
$9.0V−8.5V=0.5V$
Hence the internal resistance can be found,
$0.5V=0.425A×R$
$R=1.176Ω≈1.2Ω$