This is one of my favorite.

**Condition**:

Subtraction of two double digits number where the first and second digit of the first number equal to the second and first digit of the second number.

Example:

### Method:

For the minuend, tens digit subtract by the ones digit and then multiply by 9.

Example:

85 - 58 = (8 - 5) x 9 = 27

64 - 46 = (6 - 4) x 9 = 18

71 - 17 = (7 - 1) x 9 = 54

91 - 19 = (9 - 1) x 9 = 72

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