What are the three longest wavelengths for standing sound waves in a 124-cm-long tube that is (a) open at both ends and (b) open at one end, closed at the other?

Answer 1

For a tube open at one end, as in (b), there must be an odd number of quarter-wavelengths, if standing waves are to occur. See
http://www.tutorvista.com/content/physics/physics-iii/waves/vibrations-in-pipes.php

For a tube open at both ends, as in (a), there must be an even number of quarter-wavelengths.