For a transvers wave travelling, along a straight line, the...

Trough to crest distance

$1 .5 = \left(2{n}_{1} + 1\right)\dfrac{\lambda}{2}$ ......$( 1 )$

Trough to trough distance

$5 = \left({n}_{2}\lambda\right)$ .....$( 2 )$

from $( 1 )$ and $( 2 )$

$\dfrac{1 .5}{5} = \dfrac{2{n}_{1} + 1}{2\left({n}_{2}\right)}$

$3{n}_{2} = 10{n}_{1} + 5$

${n}_{1}$ and ${n}_{2}$ are integer

$( 1 )$ ${n}_{1} = 1, {n}_{2} = 5, \lambda = 1$

$( 2 )$ ${n}_{1} = 4, {n}_{2} = 15, \lambda = 1 / 3$

$( 3 )$ ${n}_{1} = 7, {n}_{2} = 25, \lambda = 1 / 5$