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One character of a formula becomes a blank.
The position of a blank is never the same.

Mode
1. 1 digit and 1 digit calculate. Selection from among the four.
2. 1 digit and 1 digit calculate. Direct input.
3. 2 digit and 2 digit calculate. Selection from among the four.
4. 2 digit and 2 digit calculate. Direct input.

2 digit calculate is difficult. Do you answer correctly?