Am I missing something?
It is implied, though not entirely clear, whether the jewelry box with the silver lid is one item, or two. If it is one item, then the ring is worth 10 pounds, as 190 + x + 200, and x is 10. If the jewelry box and the silver lid are two separate items, it is not clear what the ring of the silver lid are worth, as there is one equation with two variables x + y + 190 = 200. If there were two rings of equal value, than each ring would be worth 5 pounds...
Than, of course, as another mentioned, there is the issue that it is often the case, especially at auctions, that a package deal contains a discount on the individual pieces, so in this scenario, the value of the ring would be impossible to exactly determine, though perhaps it could be approximated.