



 
Suppose the wishing well has n sides. I drew a picture assuming n = 8 and I'll use it to develop a formula for the angle that will work for any n. Since there are n sides the measure of the angle BCA is 360/n degrees. The measures of the angles CAB and ABC are equal and the sum of the angles of a triangle is always 180 degrees and hence the measure of the angle ABC is
This simplifies to
Thus for example if n = 8 then the measure of the angle ABC is 90  180/8 = 90  22.5 = 67.5 degrees. I hope this helps,  


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