For a certain organ pipe, three successive resonance frequencies are observed at 425,  595, and 765, Hz respectively, Taking the speed of sound in air to be 340 m/s the fundamental frequency of the pipe (in Hz) is

We don't know, which organ pipe is used for it , so, first of all we should take ratio of all three harmonics.

e.g., 425Hz : 595Hz : 765 Hz

⇒5 × 85 Hz : 7 × 85Hz : 9 × 85 Hz

⇒ (2 × 2 + 1) 85Hz : (2 × 3 + 1) 85Hz : (2 × 4 + 1) 85Hz

what you saw, each the successive harmonics is an odd number.

This is possible only when closed organ pipe is closed .

And 85 is common for all harmonics , so fundamental frequency is 85Hz