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Explanation:
Consider a full circle with diameter 15 inches
The circumference, aka distance around the circle's edge, would be...
C = pi*d
C = 3.14*15
C = 47.1
However, we want only half the curved distance as the diagram shows.
C/2 = (47.1)/2 = 23.55
The curved distance is roughly 23.55 inches.
Then we add on the two exterior sides of 15 inches each to complete the full perimeter of the shape: 23.55+15+15 = 53.55
Notes: