दो क्रमिक श्रृंगों (crests) के बीच की क्षैतिज दूरी को निम्नलिखित में से क्या कहा जाता है?

When a transverse wave strikes normally against a rigid wall and gets reflected, then:

The fundamental frequency of a closed organ pipe is equal to the frequency of the first overtone of an open organ pipe of length 60 cm. The length (in cm) of the closed organ pipe is: