How do you multiply a two-digit number by 11 in your head?

Split the two digits apart and drop their sum in the middle — carrying if that sum reaches 10.

Because ×11 = ×10 + ×1, the middle digit becomes the sum of the outer two:

• 25 × 11 → 2 (2+5) 5 → 275.
• 72 × 11 → 7 (9) 2 → 792.

The carry pitfall (the most mis-stated trick online): if the digit sum is 10 or more, carry the ten leftward. 85 × 11 → 8 (13) 5 → the 13 gives 3 with 1 carried: (8+1) 3 5 → 935, not 8135.