Checking calculation method of second and third digit addition in second grade of primary school

A simple method is to take the nearest constant of these numbers (10 multiple):

For example, 12+27 can be understood as: (10+2)+(30-3) = 40-1= 39.

More importantly, I need to tell you that everyone's understanding of "number" and intuitive thinking are different. Some people may have special feelings about multiples of 3, 5, 2 and 10, so they choose this as the base for mental arithmetic and verification. Goodluck

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