Compute the quantity remaining after a given elapsed time, based on a substance's half-life.
Half-life and elapsed time must use the same unit (e.g. both in years, or both in days).
The formula divides elapsed time by half-life, so the units need to cancel out. If half-life is in years, elapsed time must also be in years — mixing units (e.g. days vs. years) gives a meaningless result.
It's simply elapsed time divided by the half-life. A value like 2.5 means two and a half full half-life periods have passed, and the remaining-quantity formula handles that fractional exponent smoothly.
It's a quick reference of how much of your initial quantity remains after 1, 2, 3, 5, and 10 half-lives, so you can see the decay curve at standard checkpoints beyond just your entered elapsed time.