Standing waves question with harmonics?

[Kratos321]

Homework Statement

A person hums into the top of a well and finds that standing waves are established at frequencies of 42, 70 and 98 Hz. The frequency of 42 Hz is not necessarily the fundamental frequency. The speed of sound is 343m/s. How deep is the well?

Homework Equations

just harmonic ratios.

The Attempt at a Solution

i've no clue pls help. since we can't assume that 42Hz is the fundamental.

my first approach was to use the fundamental frequency. find the wavelength and then half of that would be the depth but i don't know how to do it if we can't assume.

 

[vela]

Hint: Consider the ratio of one frequency to another, for all pairs.

 

[Kratos321]

is this system open closed?

 

[Kratos321]

ok so i figured out that the 42Hz is the third harmonic. now what shall i do?

 

[vela]

How did you figure out it was the third harmonic?