The voltage across each vertical section (ie between the horizontal lines) must be 25V. From this you can work out the current through each resistor (+ for down, - for up).
eg The voltage across the 10$\Omega$ resistor must be 30V (because 30-5=25) so the current is +3A (down).
Then add all the currents to get the current through the 25V cell. Power supplied is voltage of cell x current through it.