Since we don't know the length of the wiper blade, we can only give an expression for the final answer.
The angle of the wipers path is 120 out of an entire circle of 360. So it covers 120/360 or 1/3 of the circle.
Just take the area of a complete circle, using the blade length as the radius and multiply it by 1/3.
Input your length for r below and you will have the answer:
