$\text{Diode is in forward bias so it will offer zero resistance}$

$ R_{eq} = 5\Omega + 10\Omega = 15\Omega$

$\Rightarrow I = \frac{30}{15} = 2A $

$\text{ 2A will get distributed equally between two }10\Omega \text{ resistances}$

$ V_{AB} = 10\times 1 = 10V$