Problem: Working for a small gold mining company, you stumble across an abandoned mine shaft that, because of decaying wood shoring, looks too dangerous to explore in person. To measure its depth, you employ an audio oscillator of variable frequency. You determine that successive resonances are produced at frequencies of 31.88 and 53.13 Hz.
Part A: Estimate the depth of the shaft.
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I want to know how to work this out step by step, not just the solution please.