What it teaches
Adding and taking away inside a hundred. The jump from twenty is not a jump in size, it is a jump in method: 34 + 20 is done by moving tens, and 34 + 28 is done by moving tens and then dealing with what is left over.
This is the klasa II range, and the year in which a child stops treating a two-digit number as one lump and starts seeing the tens and the units inside it.
What to look for
The tell is where a child slows down. A pause on 40 + 30 means the tens are not yet automatic. A pause only on 47 + 38 means the tens are fine and the carrying is not — a different problem with a different fix.
Children often meet this range before they are fluent inside twenty. If the misses are frequent, going back to Adding and taking away to 20 for a week is not a step backwards, it is the shorter road.