Aseries circuit has two 5.0 on resistors and a power source of 9.0 volts. how will the current be affected and what will be another resistor (with a resistance of 7.5 ohms) is added to the circuit?

The total resistance in a series circuit is just the sum of all the individual resistors in the circuit.

Before the new resistor is added, the total resistance in the circuit is

             (5 ohms) + (5 ohms) = 10 ohms .

The current in any circuit is

               (voltage of the power source) divided by (total resistance) .

Right now, that's    (9 volts) / (10 ohms)  =  0.9 Amperes.

When the new resistor is added to the series circuit,
the new total resistance is

                     (5 ohms) + (5 ohms) + (7.5 ohms)  =  17.5 ohms  .

The new current is

                     (9 volts) / (17.5 ohms) =  0.514 Ampere  (rounded)

The addition of the extra resistor caused the current to decrease
by  (0.9 - 0.514) =  0.386 Ampere.

That's a decrease of  42.9%  of the original current.