Wandering through the streets of Confoundryville, I decide to visit an old friend who lives here. I quickly locate the street he lives in. But all houses here look the same and I cannot remember the number of his house.

The street has 50 houses where all houses numbered from 1 to 20 are on the left and the houses numbered from 21 to 50 are on the right side.

Fortunately, there is a man standing in the street. So I ask him if he knows in which house my friend lives. Being in Confoundryville he does not answer directly but says:"Why don't you try to guess it?"

Playing along, I ask the following questions:

• "On which side of the street is the house?"
• "Is the number of the house even?"
• "Is the number a square?"

I get my answers and say: "I'm still not sure, but if you tell me if the digit 4 is in the number, I will know where to go."

After getting another answer I go to the house in which I think my friend is living. I knock, a woman opens the door and it turns out that I am at the wrong house. She looks at the man who answered my questions and grins:"You asked this guy for directions? Ha! He is a Grubble. And as you may know, Grubbles always lie!"

I think about this for a moment and say:"Thanks a lot. I know the real address of my friend now"

In which house does my friend live?