Problem
Each dip wets a length equal to the water depth: 4/7 m from either end.
Same pond, same depth, so each end gets wet over the same 4/7 m length.
4.OA.A.3Draw A DiagramThe wet parts from each end overlap in the middle — the overlap is 3/7 m.
The middle stretch counted in both dips is the 3/7 m wet-twice part.
4.NF.B.3Draw A DiagramTotal length = left wet + right wet - overlap = 4/7 + 4/7 - 3/7 = (4+4-3)/7 = 5/7 m.
Adding both wet parts double-counts the middle, so subtract the 3/7 overlap once.
4.NF.B.3Count The ComplementThe pole's whole length equals the two wet lengths added together, with the wet-twice overlap taken away once.
Adding the two wet lengths lays down the whole pole but stacks the middle stretch twice, so taking that middle away once leaves the true length.
The pole splits into three touching pieces with no gap or overlap — a dry-once left end, the wet-twice middle, and a dry-once right end — so those three pieces add back to the whole pole.
Each wet length covers one end piece together with the whole middle, so the two wet lengths together hold the middle two times.
A wet region is one solid stretch cut into its end piece and the middle with nothing left out, so its length is those two pieces added.
Removing one copy of the middle cancels the extra one that the adding put in, because subtracting reverses that addition.
This only needs Grade 4 fraction add/subtract — draw the wet parts from each end and subtract the wet-twice overlap once!