I doubt she has been taught to repeatedly take off 10x8s. I find it more likely she been taught that it doesn't matter what she takes off: as long as she gets

I doubt she has been taught to repeatedly take off 10x8s. I find it more likely she been taught that it doesn't matter what she takes off: as long as she gets to the end without error, her answer will be correct. I imagine that she has reasoned that taking off 50x8 is difficult because she doesn't yet have instant access to 5x8 (which would be rare for her age group) but she can confidently subtract 8s without error (also rare). Perhaps she reasoned this earlier and has been consistently using an inefficient version of the method when she doesn't need to. In that case, she will likely be able to fix this herself when she realises. [Edited to add: I interpolate this because I am a schoolteacher who teaches math. It's a big part of my job to recognise what support a student needs: here, most likely she needs 8-times-table support. The method that I describe is well-enough established in the school system that university entrants use it. Of course, it's possible that her teacher has taught her an inefficient method, but that would be unusual. There are bad math teachers in elementary school, but that's not a typical teaching error.]