little math puzzle. False positives

[pier4r]

Mining unobtainium is hard work – the rare mineral appears in only 1% of rocks in the mine. But your friend Tricky Joe has something up his sleeve. The unobtainium detector he’s been perfecting for months is finally ready.
The unobtanium detector reports 100% success if the mineral is contained in the rock. Otherwise it says with a reliability of 90% that the rock is a dummy.

Once found a rock that is reported as positive using the detector, you get outside the mine. Tricky Joe says "look, I don't want to go selling this thing and get 50% of the revenue. If you want I get only 200$ and then you can sell it to the market for its price (1000$)".

Do you accept the deal?

Solution: https://www.youtube.com/watch?v=1csFTDXXULY
Also brilliant.org is doing a quite nice aggressive marketing, it is a pity that they collect tons of nice problems (and tons of spam) but as every modern social network focused on "now" it is impossible to find a list of them. Either one has a lot of patience to scroll the list, or the available problems are only the last ones. And once again plenty of interesting contributions are practically lost.