Answer
$R = 25~\Omega$
Work Step by Step
We can find the equivalent resistance of the three resistors:
$R_{eq} = \frac{V}{I}$
$R_{eq} = \frac{5.0~V}{0.10~A}$
$R_{eq} = 50~\Omega$
We can find the value of $R$:
$R_{eq} = 10~\Omega + 15~\Omega + R$
$R = R_{eq} - 10~\Omega - 15~\Omega$
$R = 50~\Omega - 10~\Omega - 15~\Omega$
$R = 25~\Omega$
