Exercise 3 solution

250 x 0.92 = 230 made by experienced foragers.
250 x 0.64 = 160 made after discovering a food source.
250 x 0.71 = 177.5 occurred before the first five foraging flights (about 178; a whole number of signals cannot be 177.5, so the real percentages must have been rounded).

Why you cannot add them: the three percentages answer different questions about the same signals (who made them, why, and when in the day). A single signal can be in all three groups, so the groups overlap and the sum (567.5) is not a total of anything. It is more than 250, which shows the overlap.