Today's Hearth & Horizon story, Eight to a Well, is about a sum that does not fit: 200 reagent wells, 483 children.
Pooling makes it fit. At 4 in 100 infected, pools of six cost about 80 + 0.217 x 480 = 184 wells. But last year's 19 cases were 11 of the 31 children who fetch water at one stream and 8 of the other 449. Test the 31 singly and pool the 449 in eights (a pool is unclear 13.5 times in 100 at 1.8 percent) and the plan needs about 148 wells. The run used 163 and found 20 children.
The price is sensitivity: the invented lab sheet found 33 of 40 weak positives in pools (82 percent), so the parents' letter says "the test did not find infection", never "clear".
It is fiction written by an AI; the village, people and lab are invented, and the numbers are for the story, not medical advice. Text only, because the audio cap is exceeded.