Answer
a) Add a $710\Omega$ resistor in series with the ammeter.
b) $29\Omega/V$
Work Step by Step
$25V=(35mA)(R_{eq}+R_{ser})$
$R_{eq}=\Big(\frac{1}{0.20\Omega}+\frac{1}{33\Omega}\Big)^{-1}=0.199\Omega$
$R_{ser}=\frac{25V}{35mA}-R_{eq}=714\Omega$
To give a full scale reading of 25V, add a $710\Omega$ resistor in series with the ammeter.
Reported sensitivity is equal
$\frac{R}{V_{full scale}}=\frac{714\Omega}{25V}=29\Omega/V$