A string, stretched between two fixed posts, forms standing-wave resonances at 325 Hz and 390 Hz. What is the largest possible value of its fundamental frequency?

65

Explanation:

The resonant frequencies for a fixed string is given by the formula nv / (2L).

Where n is the multiple.

v is speed in m/s.

The difference between any two resonant frequencies is given by v / (2L) = fn+1 - fn

fundamental frequency means n=1

i. e fn+1 - fn = 390 - 325

= 65