Answer
$r = 10\Omega$
$\varepsilon = 60V$
Work Step by Step
let $r$ be the internal resistance of cell and $\varepsilon$ be the emf of cell
current is given as
$I =\frac{\varepsilon}{R+r}$
when $R=0$
$6A =\frac{\varepsilon}{r}\space....................................(i)$
when $R=20\Omega$
$2A =\frac{\varepsilon}{20\Omega+r}\space...................................(ii)$
dividing $(i)$ by $(ii)$, we get
$ 3 = (20\Omega +r)/r$
$3r -r =20\Omega$
$r = 10\Omega$
putting value of $r$ in $(i)$, we get
$\varepsilon = 6A* r = 6A* 10\Omega$
$\varepsilon = 60V$
