10块钱去了哪里？

读毕下文后，请您告诉我：
10块钱去了哪里？
Ten Dollars ????????
This is very interesting question,try to figure out answer
Three Ladies went to a hotel to share a room.
The clerk asked for $300.
Each Lady paid $100, making up the $300.
The hotelkeeper then decided to allow a discount for the day, charging only $250 for the room.
He told the clerk to return $50 to the three Ladies.
The clerk pocketed $20 for himself. He gave the remaining $30 back to the three Ladies.
Each Lady took back $10. Therefore, each Lady paid 100 – 10 = $90 to the hotel.
$90 x 3 = $270 + the clerk’s $20 = $290.
Question: Where has the remaining $10 gone?
There is widespread interest in this question posted by a university in N.Z.
Many people are still trying to work out an answer.
这是个十分有趣的数字问题，使不少的人算了又算，在找答案。好吧，让我们也来“伤脑筋”。
                      常在意 拾趣 18/2/2013
“拾趣”是我看到或听到而整理改写的故事，希望能博您一笑。