An investigation involving adding and subtracting sets of consecutive numbers. Lots to find out, lots to explore.
Choose any three by three square of dates on a calendar page...
Investigate how you can work out what day of the week your birthday will be on next year, and the year after...
The numbers along the leading diagonal total 58, which is
therefore the sum of each row and each column. We can now calculate
that the number to the left of 10 must be 20 and below that must be
7. Hence x=21
This problem is taken from the UKMT Mathematical Challenges.