Brainteasers

Brainteasers are becoming increasingly popular these days. You hear them from friends, in bars and on blogs, and they are even used in job interviews to stump candidates. Some are easy, some are tricky, and some are really hard.

A common teaser for example is:

"A melon weights 1200gr and contains 99% water. Suppose that after one week the amount of water has reduced to 98%. What is the new weight of the melon?"

Solution:
It is 600gr! Just observe that the $1200$gr is made up of  $99\%$ water and $1\%$ melon-meat. That are $1188$gr water $12$gr melon-meat. After one week the weight of the melon reduces the $x$gr, whereof $y$gr are water and still $12$gr melon-meat. And it is known that the $y$gr water are now $98\%$, hence the $12$gr melon-meat must be $2\%$. But if $2\%$ are $12$gr, then $100\%$ are $600$gr.