[OT} Puzzle thats driving us nuts

Yowie wrote:
For Christmas, my parents were given a Mensa calender with a puzzle for each
day. They haven't opened it yet, but the sample question was:

A toy shop prices toys on the whim of the owner:
A doll is $12, a kite $15, a skateboard $34, and a ring toss $24. How much
is a blackboard?

My whole family has been working furiously on this puzzle, but even though
we know the answer (it was printed on the box) we have no idea *why* $31
right

Can anyone out there in RPCA land explain the solution? It will save the
Greater Chapman Clan from going utterly bonkers.

Yowie

I can't wait to hear explanations for this one. Personally, my thoughts
are a)that it says "on the whim", therefore one would not expect a
pattern, and b) because the owner said? ;-)

- Denise Brennasmeowmy