Simple Math Puzzle | Same Last Digit

I discovered an interesting sequence of math operation the other day when I was supervising some students. The results of the math questions always come with the same number regardless whether doing addition or subtraction. Initially, I was puzzled with the answers but a bit of thinking revealed the reason.

Below are samples of the math questions.

The questions:

1a) 6 - 5 = 1

1b) 6 + 5 = 11

2a) 8 - 5 = 3

2b) 8 + 5 = 13

3a) 10 - 5 = 5

3b) 10 + 5 = 15

Notice that the pair of results for the respective a) and b) having the same last digit.

Can you explain why?

I like to hear your answer in the comment section.

Happy thinking...... :)

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