Some Number Magic?

This is a great way to practice addition and subtraction of two digit numbers.  I have found students will stick at a task like this for longer than doing a page of exercises from a Math(s) Book

•Choose any number between 50 and 99
                                                                                                        75
•Add 62 to this number
                                                                                                         75 + 62 =137
•Cross off the left hand digit (hundreds digit) of the
result and add it to the units digit
                                                                                                         137; 37+1; 38
•Subtract the result from your original number
                                                                                                          75-38=37
•Try it again with a different starting number

•What do you notice?

•Can you think why this always happen?

•Now work out how you can choose a number
(between 50 and 99) and add a special number, so
that when you cross off the left hand digit (hundreds
digit) of the result and add it to the units digit and then
subtract the result  from your original number you will arrive at 45.