Applying a range of strategies to mentally calculate subtraction

Maths

To do something well without wasting energy.

Changing the order of the addends to make a calculation easier

Which of these is not a strategy for subtracting efficiently?

When calculating 50 - 48 which is an efficient strategy?

When calculating 92 - 57 which would be an efficient strategy?

Which of these is not a strategy for adding efficiently?

Counting on in my head from the largest number

When adding 66 and 67 which is an efficient strategy?

When adding 69 and 81 which would be an efficient strategy?