Open the link for circuit diagram

http://i864.photobucket.com/albums/a...me/circuit.jpg

Keeping in view the circuit provided in the link if we find the equivalent resistance of the circuit by using the formula

Equivalent resistance = open circuited voltage/short circuited current

Equivalent resistance = 3V/0.4mA = 7.5kohm

But when we generally calculate the equivalent resistance then the 30kohm resistance and 10kohm resistance are in series because they share only a single node and in this way equivalent resistance comes out to be

Equivalent resistance = 30k + 10k = 40kohm

Why is this difference?