You can
put this solution on YOUR website! Every 2 digit number is greater than 9 and less than 100
Let M and N be two two-digit numbers, then
9 < M < 100
9 < N < 100
We add those two inequalities term by term:
9 < M < 100
9 < N < 100
------------
18 < M+N < 200
So their sum is always greater than 18 and less than 200,
so their sum can never be a 4 digit number. That's because
4-digit numbers are all greater than 999.
Edwin
