Eleven-Minute Drill

Grab any two-digit number and multiply by 11 in your head. Race yourself on license plates until any pair of digits lands in under three seconds.

Then flip the problem and judge divisibility by 11 with the alternating sum. The year 1,925,462 gives 1-9+2-5+4-6+2 = -11, a multiple of 11, confirmed. A common puzzle asks whether 123,456,789 is divisible by 11: its alternating sum is 5, so no.

Run both drills for one week, five minutes a day. The 11s stop feeling like a trick and start feeling like recognition.