Spectrum #79: Order these by half-life — the solution
The Spectrum puzzle from November 1, 2026: six things on one hidden scale, shortest to longest, with every figure and where the checks were best spent.
The puzzle
Order these by half-life, shortest at the top and longest at the bottom. The board opened in this order:
- Carbon-14
- Plutonium-239
- Iodine-131
- Tritium
- Radon-222
- Uranium-238
Uranium-238 was already in place; Radon-222 opened 4 places from home.
The walkthrough
The ends first. Radon-222 is the shortest of the six at 0.01 years and Uranium-238 the longest at 4,500,000,000 years. Anchor those and the middle four are what is left to place.
Where the scale splits. The widest gap is between Plutonium-239 (24,100 years) and Uranium-238 (4,500,000,000 years) — 4,499,975,900 years apart. Everything above that line belongs on the shortest side and everything below it on the longest side, so sorting the six into those two groups is most of the puzzle.
The closest call. Radon-222 (0.01 years) and Iodine-131 (0.02 years) are only 0.01 years apart — the pair that is a coin flip without the figures, and the place to spend a check: a check locks whatever is already right and leaves only the wrong tiles to swap.
The answer
| # | Shortest to Longest | Figure |
|---|---|---|
| 1 | Radon-222 | 0.01 years |
| 2 | Iodine-131 | 0.02 years |
| 3 | Tritium | 12.3 years |
| 4 | Carbon-14 | 5,730 years |
| 5 | Plutonium-239 | 24,100 years |
| 6 | Uranium-238 | 4,500,000,000 years |
Previous: Spectrum #78 (Sat, Oct 31) · Next: Spectrum #80 (Mon, Nov 2) · All Spectrum solutions
About Spectrum
Spectrum is Noggin’s daily ordering puzzle: six things, one hidden scale, three checks. Tiles in the right place lock green; the figures turn over when the board is done. How to play Spectrum, or play today’s.