Maths - subtraction

This week we have been revising our addition, subtraction, multiplication and division. For home learning this week, I have the problem 57 - 29 =? Here are my two strategies: place value and reversibility.

Reversibility

57 - 29 = ?
29 + __ = 57
29 + 1 = 30
30 + 20 = 50
50 + 7 = 57

I need to add 1, 20 and 7 because they're the numbers I added on above.

1 + 20 + 7 = 28  therefore the answer of the question 57 - 29 = 28

Place Value

57 - 29 = ?
57 - 20 = 37
37 - 9 = 28

So the answer to the question is 57 - 29 = 28