The 2nd paragraph has the straightforward answer, technically.
This is an example of gross domestic product and today's marketing. When a product is usually paid more money to manufacture than it costs to actually sell. However, the usual difference between manufacturing cost and sales cost is on average is at most 5 - 15% of the manufacturing cost.
The second problem however is just the perfect example of sales and sales tax. The diamond ring is on sale for 30% off in this case, because 290 divided by its percent is equal to 30%, the remaining amount.
