resonance and tubes

m.l.

1. The problem statement, all variables and given/known data
An open vertical tube is filled with water and a tuning fork vibrates over its mouth. As the water level is lowered in th etube, resonance is heard when the water level has dropped 17 cm, and again after 49 cm of distance exists from the water to the top of the tube. What is the frequency of the tuning fork?


2. Relevant equations
lamda=2xlength, f=v/ lamda, speed of sound is 343m/s


3. The attempt at a solution
49-17=35

lamda=2l (or is it lamda=4l cause its a closed system?)
=2x.35
=0.70m

f=v/lamda
=343m/s/0.70m
= 4.9x10 exponent 2 Hz?

LowlyPion

Isn't 49 - 17 = 32 ?