Explanation

(b) In a closed organ pipe in which length of air-column can be increased or decreased, the first resonance occurs at $\lambda/4$ and second resonance occurs at $3\lambda/4.$
Thus, at first resonance
$\frac{\lambda}{4} = 13\ \ \ \ \ \ \ \ \ldots(i)$
And a second resonance
$\frac{3\lambda}{4} = 41\ \ \ \ \ \ \ldots(ii)$
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Subtracting Eq.(i) from Eq.(ii), we have
$\frac{3\lambda}{4} - \frac{\lambda}{4} = 41 - 13$
$\Longrightarrow \frac{\lambda}{2} = 28$
$\therefore\ \ \ \ \lambda = 56\ cm$
Hence, frequency of tuning fork
$v = \frac{v}{\lambda} = \frac{350}{56 \times 10^{- 2}} = 365\ Hz$