Well I've been pretty busy recently and I haven't been on the board as much as I would like - particularly to answer some of the questions recently posted.

But anyway, I received this puzzle from someone the other day. Take a look:

An absent-minded mathematician has been asked to call at a house in a street (in which the houses are numbered consecutively from 1 upwards in the usual fashion), only he has forgotten its number. What he does remember, though, is that the sum of the house numbers before this house happens to be equal to the sum of the house numbers after this house, and also that the house he is looking for has a two-digit number.

What is the number of the house he is looking for? And how many houses are there in the street?

Only pencil and paper are permitted.