This is the exact position of the rings after the 9,999th move has been made, and the reader will find that the method shown will solve any similar question, no matter how many rings are on the tiring-irons. But in working the inverse process, where you are required to ascertain the number of moves necessary in order to reach a given position of the rings, the rule will require a little modification, because it does not necessarily follow that the position is one that is actually reached in course of taking off all the rings on the irons, as the reader will presently see. I will here state that where the total number of rings is odd the number of moves required to take them all off is one-third of (2(n + 1) - 1).
With n rings (where n is odd) there are 2n positions counting all on and all off. In 1/3 (2(n+1) + 2) positions they are all removed. The number of positions not used is (1/3)(2n - 2).
With n rings (where n is even) there are 2n positions counting all on and all off. In (2(n + 1) + 1) positions they are all removed. The number of positions not used is here (1/3)(2n - 1).
It will be convenient to tabulate a few cases.
| No. of Rings. | Total Positions. | Positions used. | Positions not used. |
| 1 | 2 | 2 | 0 |
| 3 | 8 | 6 | 2 |
| 5 | 32 | 22 | 10 |
| 7 | 128 | 86 | 42 |
| 9 | 512 | 342 | 170 |
| 2 | 4 | 3 | 1 |
| 4 | 16 | 11 | 5 |
| 6 | 64 | 43 | 21 |
| 8 | 256 | 171 | 85 |
| 10 | 1024 | 683 | 341 |
Note first that the number of positions used is one more than the number of moves required to take all the rings off, because we are including "all on" which is a position but not a move. Then note that the number of positions not used is the same as the number of moves used to take off a set that has one ring fewer. For example, it takes 85 moves to remove 7 rings, and the 42 positions not used are exactly the number of moves required to take off a set of 6 rings. The fact is that if there are 7 rings and you take off the first 6, and then wish to remove the 7th ring, there is no course open to you but to reverse all those 42 moves that never ought to have been made. In other words, you must replace all the 7 rings on the loop and start afresh! You ought first to have taken off 5 rings, to do which you should have taken off 3 rings, and previously to that 1 ring. To take off 6 you first remove 2 and then 4 rings.
[418.—SUCH A GETTING UPSTAIRS.—solution]
Number the treads in regular order upwards, 1 to 8. Then proceed as follows: 1 (step back to floor), 1, 2, 3 (2), 3, 4, 5 (4), 5, 6, 7 (6), 7, 8, landing (8), landing. The steps in brackets are taken in a backward direction. It will thus be seen that by returning to the floor after the first step, and then always going three steps forward for one step backward, we perform the required feat in nineteen steps.