Final answer:
The overall efficiency of two control devices in series, with efficiencies of 90% and 95%, is calculated by multiplying their efficiencies together, resulting in an overall efficiency of 85.5%.
Step-by-step explanation:
To find the overall efficiency of two control devices placed in series, you multiply the efficiencies of the individual devices. The first device has an efficiency of 90%, or 0.90, and the second device has an efficiency of 95%, or 0.95. To calculate the overall efficiency, use the following equation:
Overall Efficiency = Efficiency of Device 1 × Efficiency of Device 2
Plugging in the values:
Overall Efficiency = 0.90 × 0.95 = 0.855 or 85.5%
The overall efficiency of the system is thus 85.5%, which corresponds to option A. This efficiency represents the percentage of input that is converted to useful output without being lost as waste.