An organ pipe is closed at one end has fundamental frequency of 1500 Hz. The maximum number of overtones generated by this pipe which a normal person can hear is:(Audible frequency range is 20Hz to 20KHz)  

Critical hearing frequency for a person is 20,000Hz.
If a closed pipe vibration in N^th mode then frequency of vibration$\mathrm{n}=\frac{(2\mathrm{N}-1)\mathrm{v}}{4\mathrm{l}}=(2\mathrm{N}-1){\mathrm{n}}_1$
(where n₁ = fundamental frequency of vibration) 
Hence 20,000 $=(2\mathrm{N}-1)\times 1500\Rightarrow \mathrm{N}=7.1\approx 7$
Also, in closed pipe 
Number of over tones = (No. of mode of vibration) –1 = 7 – 1 = 6.