Answer:
Divide the given diameter by 2 to obtain the radius in feet.
Divide the radius in feet by 3 to obtain the radius in yards.
Square the radius and multiply the result by an approximation of  suitable for your required level of precision. This calculation will provide you with the area of the circle in square yards.
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Multiply the calculated area by 13 and round to the nearest whole number. (unless you think the hero of this story knows of a nursery that will sell him fractional parts of a plant)
Notes on approximation of  for this problem. Either , , or any representation of  to greater precision will result in the same number of plants after rounding to the nearest whole number of plants.
John
Step-by-step explanation: