Answer
$R = 9950\Omega$
Work Step by Step
resistance of ammeter is $50\Omega$.
when potential difference is $5.0V$ and meter goes to full scale that means current is $500\mu A$. thus we have
$ \Delta V = IR + I*50\Omega$
$5V = 500\mu A * (R+50\Omega)$
$R+ 50\Omega = 10000\Omega$
$R = 9950\Omega$
