Is a bracelet marked 18K and 0.750 Gold?

18K has the same meaning as 0.750. It means that 18 parts out of 24 parts (18/24=0.75) are (supposed to be) gold. Just because something is marked with 18K, doesn't mean that it is real. If it is real gold, then 18K (also 0.750 or 750) means that the gold on the item consists of 75% pure gold. "On the item", because gain, just because it actually is real gold, it doesn't necessarily mean that the whole item is made of it, but it could be just plated.