A close wavelength is not accepted unless equality closes. 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 nearby wavelength still has to close the equality
The path from one wavelength is 4 meters, but two plane spacings give 6 meters.
nλ=1⋅4=4m
The rejected bit is computed, not captioned
Because 4 meters is not 6 meters, the accepted bit is 0.