I think it might be what's called chunking here, but don't quote me! Very sensible way to do subtraction - see how far it is from the number being subtracted to the nearest convenient number in the right direction, e.g. multiple of 10, add that to how far it is from the convenient number to the thing being subtracted from.

Playing around with number patterns, finding quick or just different ways to do sums, and working out whether they always work, is good stuff - encourage!

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