If each diode in figure has forward bias resistance of $25\Omega$ and infinite resistance in reverse bias , what will be the value of current $I_1,I_2,I_3$ and $I_4$ ? 

$I_3$ is zero as the diode in that branch is reverse biased. Resistance in the branch AB and EF are each $(125+25)\Omega =150\Omega$ 

As AB and EF are identical parallel branches , their effective resistance is $\frac{150}{2}=75\Omega$ 

$\therefore$ Net resistance in the circuit = $(75+25)\Omega =100\Omega$ 

$\therefore$ Current $I_1=\frac{5}{100}=0.05A$ 

AS resistance of AB and EF are equal and $I_1=I_2+I_3+I_4,I_3=0$ 

$\therefore I_2=I_4=\frac{0.05}{2}=0.025A$