[mjaniczek]

I wonder how does that work. Why can you use the 5 flies you found to divide the total sample size?

It's kind of giving you an invariant number: * 100 flies -> 5 flies (5%), 100 / 0,05 = 2000 * 80 flies -> 4 flies (5%), 80 / 0,04 (not 0,05) = 2000

But if you used the percent instead (which seems to be what you hinted at) then the results vary: * 100 flies -> 5 flies (5%), 100 / 0,05 = 2000 * 80 flies -> 4 flies (5%), 80 / 0,05 = 1600

What's the principle, does it have a name I could read more about on Mathworld / Wiki?

[m12k]

You can read more about the technique here: https://en.wikipedia.org/wiki/Mark_and_recapture