Diameter = 2 yd
Radius = 1 yd
Circumference = pi D = 2 pi = 6.283 yd(rounded)
Area = pi R2 = pi (1)2 = 3.142 yd2 (rounded)
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You neglected to say whether the 4mm is the radius of the circle, or the diameter,or the circumference. Each possibility leads to a different area.
To find the circumference of a circle then multiply pi (3.14159265...) by the diameter of the circle. To find the area then multiply pi by the radius squared.
They would have to give you enough information to eventually get the diameter. If they gave you the area of a circle and asked for the circumference, you can determine the radius, and from the radius the diameter, and then the circumference. If they said "What is the circumference if the area of a circle is 36pi?" Area of a circle is pi*r2, which would give us a value for r = 6 Diameter is 2*r, or = 12. Circumference is d*pi, or 12pi
If the circumference or diameter is given then you can find the radius or simply measure the distance from the centre of the circle to the circumference.
When the diameter of the circle doubled, its circumference also doubled.However, its area increased to 4 times the original area, while nobody was looking.