Aparallel circuit has two 8.0-ohm resistors and a power source of 9.0 volts. if a 12.5-ohm resistor is added to the circuit in parallel, how will the current be affected and what value will it have?

-- The two 8-ohm resistors in parallel are equivalent to
a single 4-ohm resistor.

-- The current in the circuit is 

                 (voltage) / (resistance) = (9v) / (4ohms) = 2.25 Amperes

-- If another resistor is added in parallel, then no matter how large
or small it is, the current in the circuit increases. 
(When another lane is added to a road, the traffic capacity of the
road increases.)

The circuit is now equivalent to a 12.5-ohm resistor in parallel
with 4-ohms.  That's ...

       (12.5 x 4) / (12.5 + 4)  =  50/16.5  =  3.03 ohms .

-- The new current is  (9v) / (3.03 ohms)  =  2.97 Amperes