2015 Q3 (Spectrometer experiment)

can someone PLEASE explain to me why in 2015 Q3 when calculating the wavelength of light in the marking scheme it says: when calculating d ( formula, d=1/n(lines per mm) ) it says to put d=1/80000, when in the question it says 'the diffraction grating had 80 lines per mm' should it not be 1/80??? as the formula is d=1/lines per mm.

Midleton— 15/05/16