Answer
$R=25\Omega$
Work Step by Step
applying kircchoffs law
$IR_1 + IR_2+IR_3 =\Delta V$
$I(R_1 + R_2+R_3 )=\Delta V$
$100mA*(10\Omega +15\Omega + R) = 5.0V$
$10\Omega +15\Omega + R =\frac{5.0V}{100mA} = 50\Omega$
$R = 50\Omega- 10\Omega-15\Omega$
$R=25\Omega$
