Found this puzzle on pzzls.com
Mr. House would like to visit his old friend Mr. Street, who is living in the main street of a small village. The main street has 50 houses divided into two blocks and numbered from 1 to 20 and 21 to 50. Since Mr. House has forgotten the number, he asks it from a passer-by, who replies "Just try to guess it." Mr. House likes playing games and asks three questions:
- In which block is it?
- Is the number even?
- Is it a square?
After Mr. House has received the answers, he says: "I still do not know, but tell me, is the digit 4 is in the number?" After hearing the answer, Mr. House runs to the building in which he thinks his friend is living. He rings, a man opens the door and it turns out that he has the wrong address. The man starts laughing and tells Mr. House: "Your adviser is the biggest liar of the whole village. He never speaks the truth!". Mr. House thinks for a moment and says "Thanks, now I know the real address of Mr. Street".
What is the address of Mr. Street?
pzzls - nice site with a good variety of math and logic puzzles, as well as some others.