A tuning fork of frequency 400 Hz is held above a glass tube of radius 1.8 cm that is closed at the…

Step 1. The speed. $L_2 - L_1 = 63.6 - 20.5$ $= 43.1$ cm $= \dfrac{\lambda}{2}$. So $v = 2f(L_2 - L_1)$ $= 2 \times 400\ \text{Hz} \times 0.431\ \text{m}$ $= 345\ \text{m/s}$.

Step 2. The end correction. $e = \dfrac{L_2 - 3L_1}{2}$ $= \dfrac{63.6 - 61.5}{2}\ \text{cm} = 1.05$ cm. The rule $e = 0.6r$ gives $0.6 \times 1.8\ \text{cm} = 1.08$ cm, close to it.

Step 3. The wrong rule. $v = 4fL_1$ $= 4 \times 400\ \text{Hz} \times 0.205\ \text{m}$ $= 328\ \text{m/s}$.

Step 4. The error. $\dfrac{345 - 328}{345} = 4.9\%$.