for an open-ended tube of length $L$ ...

$f_1 \implies L = \dfrac{\lambda}{2} \implies \lambda = 2L$

$f_2 \implies L = \lambda$

$f_3 \implies L = \dfrac{3\lambda}{2} \implies \lambda = \dfrac{2L}{3}$

$f_4 \implies L = 2\lambda \implies \lambda = \dfrac{L}{2}$

Note the sequence for values of $\lambda$ ...

$\dfrac{2L}{1}, \, \dfrac{2L}{2}, \, \dfrac{2L}{3}, \, \dfrac{2L}{4}, \, ...$