When travelling in Novaria and not understanding the currency, I offered a silver note for an item marked '87N'. I was given, in change, two copper coins and one gold coin. Copper coins are valued at the lowest. Later, for a newspaper marked '45N', I offered a handful of coins and the vendor took three copper and one gold. There are only these three denominations of money available; the smallest is marked '1N' and each higher denomination is a whole number multiple of the lower denominations. How many copper coins are worth one silver note?