Answer
$r=1.17\Omega$
Work Step by Step
Let $r$ be the internal resistance
current is given as
$I =\frac{\epsilon}{R+r}= \frac{9.0V}{20\Omega+r}$
now terminal resistance is given as
$V_t = \epsilon -Ir$
$8.5V = 9.0V - \frac{9.0V}{20\Omega+r}r$
$\frac{9.0V}{20\Omega+r}r =0.5V$
$9.0V *r = 10V\Omega +0.5V*r$
$8.5 V*r =10V\Omega $
$r= \frac{10V\Omega}{8.5V} $
$r=1.17\Omega$
