It is a 1980 Nishiki Olympic, 100% positive, based on the serial number, components and decals. The interesting aspect is the N prefix for the serial number as I have not seen an N in use before. However, this letter is a market indicator and I wonder is this bicycle was originally orignally intended for a market other than Canada or the USA, which is what I nomally see.

Regardless, the Olympic was an entry level model, typically 2nd or 3rd from the bottom of the line and should weigh about 28 lbs. Yours has a hi-tensile steel frame which is rugged and well built. The component selection is decent and appears to be all original. It's a good but not great bicycle. But at $5 it is a great purchase.