The YouTube channel Mind Your Decisions has a lot of great puzzles, like this one: Can You Solve The Magical Pond Puzzle?
Highlight to reveal solution:
The video has a solution, but mine is a little different.
I construct a reverse timeline. Below I write all my actions, and I count the flowers. For now I assume I leave 1 flower at each temple, and that fractions are okay.
| * | 0 flowers |
| * | Leave 1 flower at temple 3 |
| * | 1 flower |
| * | Swim |
| * | 1/3 flowers |
| * | Leave 1 flower at temple 2 |
| * | 4/3 flowers |
| * | Swim |
| * | 4/9 flowers |
| * | Leave 1 flower at temple 1 |
| * | 13/9 flowers |
| * | Swim |
| * | 13/27 flowers |
If I take all these numbers, they are: 0, 1, 1/3, 4/3, 4/9, 13/9, 13/27.
In reality, I don’t know how many flowers I leave each time. If I leave n flowers, my numbers would be: 0*n, 1*n, 1/3*n, 4/3*n, 4/9*n, 13/9*n, 13/27*n. All of these numbers have to be nonnegative integers. To accomplish that, n has to be a multiple of 27. To use the smallest number of flowers, n is simply 27. So we had 13 flowers to begin with.
Stuff from ommadawn.dk (Lise Andreasen) 