In Barcelona during a certain week the average daily rainfall from Sunday to Friday exceeded the average from Monday to Saturday by 16.667%.
The week was on the average just as wet as Sunday.
What was the fall on Saturday?
(In reply to How do we treat evaporation? (spoiler)
by Steve Herman)
Great start, Steve! However, your solution assumes the same rainfall each day, Sun through Fri. But that isn't necessarily so.
From fact #1: Sun + M+ ... + F = (7/6) * (M + . . . + F + Sat)
Simplifying: 6 * Sun = M + . . . + F + 7 * Sat -----------> Eq. 1
From fact #2: (Sun + M + . . . + Sat) / 7 = Sun
Simplifying: M + . . . + F + Sat = 6 * Sun -------> Eq. 2
From eq. 1 & 2: M + . . . + F + Sat = M + . . . + F + 7 * Sat
=> 0 = 6 * Sat
So, Sat = 0 (i.e., Saturday was dry.)