If a signal transmitted with a power of 200 watts is received with an S-meter reading of "10 dB over S9," what would be the new reading if power was reduced to 20 watts?

Reducing the transmitter power from 200 watts to 20 watts means the power is divided by 10. In decibel terms, a power ratio of 1/10 corresponds exactly to a 10 dB decrease in signal strength. On a receiver's S-meter, signal levels above S9 are read directly in decibels. The original received signal was "10 dB over S9." If the signal strength drops by 10 dB due to the power reduction, you subtract 10 dB from the original reading. This drops the signal precisely down to the S9 mark.