In case of open organ pipe we have all types of harmonic, even as well as odd. While in case of close organ pipe we have only odd harmonic. 

The ratio of two adjacent frequencies $\frac{{\mathrm{f}}_{\mathrm{n}}}{{\mathrm{f}}_{\mathrm{n}+1}}=\frac{550}{650}=\frac{50\times 11}{50\times 13}=\frac{11}{13}$

Hence the adjacent harmonic are 11th and 13th. Both harmonic are odd hence it is the case of close organ pipe. Clearly, the fundamental frequency should be 50 Hz.

The fundamental frequency in case of close organ pipe

${\mathrm{f}}_0=\frac{\mathrm{v}}{4\mathrm{l}}\Rightarrow 50=\frac{350}{4\times \mathrm{l}}$

Hence length of the pipe $\mathrm{l}=\frac{350}{50\times 4}=1.75\mathrm{m}$