Question 361205
If r=1 the square is 2 by 2 = 4 sq units
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The area of the circle = {{{pi*r^2}}} = pi units
The unwatered part is:
4 - pi sq units
=~ 0.86 sq units.
0.86/4 = 0.215
=~ 21.5%
Your answer is correct, and it's the same %age for any size square.
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PS  It's pi, the Greek letter, not pie, the dessert.