It's just standard D&D conversion: 10 copper = 1 silver, 10 silver = 1 gold, 10 gold = 1 plat.

Same as in the Player's Handbook: 10 copper makes 1 silver, 10 silver makes 1 gold, 10 gold makes 1 platinum. Electrum (worth 5 silver) and platinum are rarely minted by modern nations, but ancient Cael Morrow minted electrum coins.

Thanks!