The two pipes are submerged in sea water, arranged as shown in figure. Pipe A with length $L_A =1.5m$  and one open end, contains a small sound source that sets up the standing wave with the second lowest resonant frequency of that pipe. Sound from pipe A sets up resonance in pipe B, which has both ends open. The resonance is at the second lowest resonant frequency of pipe B. The length of the pipe B is :