Math Is Fun Forum

I have this problem:
I live on a street where the houses are numbered from 1 up consecutively.  The sum of all the house numbers less than mine is the same as the sum of all the house numbers greater than mine.  My house # is in the 30's.  What is the house # and how many houses are on the street?

I can get the answers of 35 and 49 thru doing the sums of all the possiblilies of the house # being 30 to 39.  But is there a formula?  We aren't allowed to use guess and check.