81-50 is the sort of calculation I used to do outside.
We put on our imaginary 10 league boots and take giant strides along a line, counting backwards in 10s.
So, boots on.
How many steps will make 50? 5 steps.
Put up 5 fingers to represent the 50 you are taking away.
81, 71, 61, 51, 41 31 ( Putting down a finger for each step)
Then we moved onto using a blank number line and drawing the steps as 5 curved lines, marking the landing points as we go.
Then a lot of practice with 100 squares, number lines and the rest.