One line of arithmetic decides the goal, and it is worth writing out because it is the whole appeal of the format. The envelopes are numbered 1 to 100, and the sum of the numbers 1 to n is n × (n + 1) ÷ 2 — so 100 × 101 ÷ 2 = 5,050. Fill every envelope and you have put away exactly that, whatever order you did it in.
The draw is what makes it survivable. Each day one envelope comes up at random from the ones still unfilled, and the number on it is what goes away that day. The shuffle on screen is suspense rather than a loading state; the number it stops on is the number that was drawn. One envelope a day is the cap, so the whole thing is 100 days at the earliest.
Money saved is not a running total the page maintains — it is computed every time as the sum of the envelope numbers you have actually filled, and money left is 5,050 minus that. That invariant is why the two figures can never disagree with the board: there is one record, the envelopes, and everything else is read off it.
The average-left line is the money still owed divided by the envelopes still unfilled, which starts at 50.50 and tells you honestly whether the big numbers are still ahead of you. Drawing at random means the average holds but the order does not: an unlucky first fortnight leaves the easy envelopes for the end, and there is nothing the tool can do about that except say so.
Undo reaches today and no further. An envelope filled yesterday is settled money and stays on the board. There is no currency here and no account — the figures are unitless amounts in whatever you are saving in.
Unbroken counts days. It does not measure anything about your body, and nothing above is medical,
nutritional or fitness advice. Where a number here is our own judgement rather than a published figure,
the note says so and cites nothing.
Method last checked 21 August 2026.
