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Surprising Split

Age 7 to 11 Challenge Level:

Here is another way of thinking about the trick...
For 54 x 11, set out a column addition with 54 as the first number to add.
Write 54 again, below the first 54, but this time line up the 4 in the ones column of the second 54 with the 5 in the tens column of the first 54.
Add them together:

column addition with 54 as the first number, then 54 written again below, but with the 5 in the hundreds column and the 4 in the tens column. The answer written is 594 


What digit is missing from the ones column of the second number in the sum?  
What do you know about numbers that end in that digit?
 
Why is this the same mathematically as the trick in the video?