Airliners taking off from City Central Airport

produce a mean noise level of 108 dB

(decibels), with a standard deviation of

6.7 dB. To encourage airlines to refit their

aircraft with quieter engines, any airliners

with a noise level above 120 dB must pay a

"nuisance fee."

a) After two years, a sample of 1000 airliners

showed that 4 were billed the nuisance

fee. Assuming that the program has been

effective, and that the standard deviation

has not changed, what is the new mean

noise level?

The answer for this part is about 102 dB but im not sure how to get that