Two ideal diodes are connected in the network as shown in figure. The equivalent resistance between A and B…

If $A$ is connected to a higher potential compared to $B$, then left diode will be in forward bias and hence will not offer any resistance also the right diode will be in reverse bias and hence it will behave as open key. Therefore, the modified circuit will be as given below.

$\therefore$ Equivalent resistance $=\frac{20}{2}+15=25 \mathrm{\Omega }$