Interference of Sound Waves

[sp00ky]

Don't really understand the first part of the question.
When 2 trumpets were sounded together, 6 beats were heard in 2s. If the frequency of one trumpet was 786Hz, what were the possible frequencies of the 2nd trumpet. I keep on gettin' 3+Hz but dunno if it's right.

[Tunguska]

3 is the beat frequency. A beat frequency is caused by two waves with similar frequencies interfering with each another. The difference in the freq.s is equal to the beat freq. So the second note had to be either 789 or 783 Hz.