Odd + 11

Fold a year down the odd+11 way: add 11 if it’s odd, halve it, add 11 again if it’s odd

Experimental

The Odd + 11 drill is new and still finding its feet. Nothing about it is locked in — it may change shape, or go away again, depending on what people make of it. If you have an opinion, tell me.

Level selection

How to play— show the rules

This drill is the odd + 11 route through the year — an alternative to dividing by 12. You are given a year; fold its last two digits down to a single working number:

Take the last two digits. If the number is odd, add 11.
Halve it. It always divides exactly — that is what the +11 was for.
If that is odd, add 11 again. Now type it in.

That last value is the answer here. Turning it into the year’s code is one more step — 7 − (n mod 7) — which is what the Year code drill is for.

The whole point is that you only ever carry one number, instead of holding three partial results to add up at the end.

Year	Odd? +11	Halve	Odd? +11	7 − mod 7
1966 (66)	66	33	44	5
1985 (85)	96	48	48	1
2024 (24)	24	12	12	2
1999 (99)	110	55	66	4

Type your answer on the keypad, or just use your keyboard — digits, backspace, and Enter to submit a single-digit answer.