It the wiper blade (and arm) made a full rotation, 2 circles would be formed.
One small circle with radius b, and a larger circle with radius a+b.
The areas of these 2 circles are respectively:
and
120° is 1/3 of a complete angle 360° which form a circle, so the areas formed by the arm of length b, and the arm + the wiper blade (a+b) are:
and 
in squared respectively.
the actual wiped area is
(in squared)
Area of the window is 54*24= 1296 (in squared)
thus, the ratio of the wiped area to the whole area of the window is 351.7/1296=0.271
Converted to percentages, this is 27.1%
Answer: 27.1%
One small circle with radius b, and a larger circle with radius a+b.
The areas of these 2 circles are respectively:
120° is 1/3 of a complete angle 360° which form a circle, so the areas formed by the arm of length b, and the arm + the wiper blade (a+b) are:
the actual wiped area is
Area of the window is 54*24= 1296 (in squared)
thus, the ratio of the wiped area to the whole area of the window is 351.7/1296=0.271
Converted to percentages, this is 27.1%
Answer: 27.1%