counfusing physics questions dealing with decibels?

Question

a noisy machine in a factory produces a decibel rating of 80dB. How many identical machines could you add to the factory without exceeding the 90dB limit set by federal regulations?

Answer 1

Decibel levels work on a logarithmic scale, so if something is five decibles greater than another, it is five time louder. Adding one more macine would increase the total decibel level to 81 dB, adding two would increase it to 82 dB. So I surmise you could have ten machines operating at 90 dB.