The answer is 3V.

 

See the two batteries have internal resistances of 2.5 each, they r same as putting them in series with the 5 ohm resistance, so that branch has 10 ohms now.

 

now the battery having 10V will drop 5V to kill the second battery so the effective pot. difference of the circuit is now 5V only, the net resistance is 10+25+15 = 50 ohms.

 

current = 5/50 = 0.1A

 

now to calcualte pot. difference between A & B, the net resistance in its path is

25 +15 = 30

 

so V= IR = 0.1 * 30 = 3V

 

thnx