Logic & numbers
It doubles daily. Full on day 20. When was it half full?
Day 19.
The answer
Day 19.
The idea
Working backward reverses the operation: undo doubling by halving.
An example
If day 20 is full, day 19 is half and day 18 is one quarter. These fractions follow from the stipulated exact doubling, not from a general prediction about real-world growth.
Try it
What fraction is filled on day 17? Show each backward step.
More context
One doubling takes half full to full. This is a constructed mathematical pattern, not a claim about biological growth.
the 102 platform · Constructed puzzle