SOLUTION: The sum of the digits of a two digit number is 11. If the digits are interchanged,the number they form decreases the number by 9. Find the number

I will do it assuming you are not taking algebra yet.  If
you're talking algebra and studying two equations with t and u,
then tell me in the thank-you note below and I'll redo it using
algebra.

Let the number be XY.   The number YX is 9 less than XY.
So if we add 9 to YX we will get XY

               YX
               +9
               XY

Since 9 is the largest digit, then unless X is 0, we will
have to carry 1.  X can't be 0, for we'd have

               Y0
               +9
               0Y  

Then we'd have 

               90
               +9
               09

which is wrong, so we must carry 1.  Let's put a 1 to carry
above the left column:

               1
               YX
               +9
               XY               

So X is 1 more than Y.  The only pair of digits where one is
1 more than the other that have sum 11 are 5 and 6.  So the
addition is:

               1
               56
               +9
               65

So the number XY is 65.

Edwin