I am not a teacher and for all of my DDs various issues it would never occur to her to do any sort of long process with 101-99. Surely the failure to see the simplicity of this question is far bigger than what method of math curriculum is in use? Is that something that is taught or needs to be taught? Surely anyone that can could knows when two numbers are two digits apart and so knows that 10-8 is 2, 9-7 is 2 and 101-99 is 2? I am genuinely curious here. Is it common to not just see how this works? I would honestly never have imagined this as a problem. Though I guess it is similar to my never having realised that children could "read" without comprehension, which I now know is a real problem but it would never have occurred to me and I still can't understand how it works.