40 ohms

Since Rt = R1 + R2 = R3

we have:

Rt = 20 ohms + 5 ohms + 15 ohms

Rt = 40 ohms

When the voltage drops across the resistors in a series circuit are

known, the total voltage may be calculated by using the formula

Et = ER1 + ER2 + ER3

= 20 volts + 30 volts + 50 volts

Et = 100 volts

The total voltage in a circuit may also be calculated by using the

formula derived from Ohm's law:

Et = It x Rt

= 1 amp X 100 ohms

Et = 100 volts