To understand that counting on from the larger number works, you have to be completely sure that addition is commutative (1+50 = 50+1). I think it's not surprising if a young child doesn't get this, or even if they behave as though they get it and later don't - I think this kind of thing can happen as understanding deepens. E.g. if you notice for the first time that 50-1 isn't the same as 1-50, you might also doubt yourself in addition for a while! I wouldn't push this.

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