A mismatched wavelength is rejected by arithmetic. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.
highlighted = computed this step
A wavelength label does not make a mode fit
Mode index 5 and wavelength 5 meters give 25 meters.
nλ=5⋅5=25m
The chain requires twice the length
Twice the chain length is 24 meters, so the fit bit is 0.