Question

If the first day of the year 1991 was Tuesday what day of the week must have been on 1st January 1998?

Answer 1

Total number of odd days from 1st Jan 1991 to 1st Jan 1998

Year	1991	1992	1993	1994	1995	1996	1997
Odd days	1 +	2 +	1 +	1 +	1 +	2 +	1

= 9 odd days
i.e. 2 odd days
Hence the day is 2 days beyond the day on 1st Jan 1991
i.e. the required day must be Thursday.