Number Puzzle #22- Deal Or No Deal -

Imagine a mine where exactly 1% of the rocks contain a small diamond embedded within them worth exactly
$1,000. You have a diamond detector that always beeps if a diamond is embedded within a rock. If there is
no diamond within the rock, 10% of the time the detector will detect a false positive (that is to say it
will beep even though no diamond is present). The owner of the mine allows you to use your device on one
of the rocks. You pick up a random, unmarked rock and the detector beeps. The owner offers to sell you
the rock for $200. If the rock contains a diamond, it's worth $1,000; if it doesn't, it's worthless.
Should you buy the rock?
ANSWER:
No.
EXPLANATION: Let's say the mine had exactly 1,000 rocks. 10 of those rocks (1%) would
contain a diamond and would set off the detector. As for the other 990 rocks without a diamond, 99 rocks
(10%) would set off the detector despite not having a diamond. In all, 109 rocks would trigger the
detector. Your rock could be any one of them. The chance of your rock containing a diamond are 10 out of
109 so the value of the rock is: (10/109 * $1,000) + (99/109 * $0) = $91.74. You should not pay $200 for
a rock that on average will have a value of only $91.74.
Do you have a
suggestion for this puzzle (e.g. something that should
be mentioned/clarified in the question or solution, bug, typo, etc.)?