Answer:
6.4 V
Step-by-step explanation:
You have a series circuit of three resistors and a 24 V power supply. You want to know the voltage across the 40 Ω resistor, when the other two resistors are 100 Ω and 10 Ω.
Voltage drop
The fraction of total voltage in a series circuit that is dropped across any resistor is the ratio of that resistor value to the total resistance in the circuit. The voltage drop across a 40 Ω resistor in a circuit with a total resistance of 150 Ω is ...
(24 V)(40/150) = 32/5 V = 6.4 V