Guess I recommend doing that
125 because f=ma so you would use 100=mx0.75
Explanation:
It is given that,
Fundamental frequency, f = 220 Hz
(a) We know that at 0 degrees, the speed of sound in air is 331 m/s.
For open pipe, 
l is the length of pipe
Also,


(b) Let f' is the fundamental frequency of the pipe at 30 degrees and v' is its speed.


v' = 348.71 m/s
So, 

f' = 232.4 Hz
Hence, this is the required solution.
Hook's law states that

where F is the applied force, k is the spring constant, and
is the change in length.
Plugging your values, we have
