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.
