"Pipe A has length L and is open at one end and closed at the other. Pipe B is open at both ends and has length 2L. Which harmonic of pipe B matches in frequency the fundamental of pipe A?"

Explanation:

length of pipe A is L

frequency of fundamental note of pipe A which is closed organ pipe

= velocity of sound / 4 x length of pipe

=  V / 4 x L where V is velocity of sound .

Similarly frequency of fundamental note of pipe B which is open organ pipe

= velocity of sound / 2 x length of pipe

= V / (2 x 2L) =  V / 4L , This is also called first harmonic

so fundamental frequency of pipe A will be equal to first harmonic of pipe B .