Maths Olympiad Prep

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, 2024

Geometry Difficulty 1.6 Junior Find the answer Canada

A circular spinner is divided into five equal sections. An arrow
is attached to the centre of the spinner and is positioned as shown.

Figure 0

The arrow is spun clockwise, and it stops in the section labelled
DD. Which of the following could
have been the angle of rotation?

Pick one

Solution

The angle of one complete rotation measures 360°\degree. Since the spinner is divided into 55 equal sections, each 360°5=72°\dfrac{360\degree}{5}=72\degree rotation moves the arrow to the next dividing line between two sections. Rotating clockwise, the section labelled DD is the 44th section. This means that if the angle of rotation is greater than 3×72°=216°3\times72\degree=216\degree and less than4×72°=288°4\times72\degree=288\degree, then the arrow stops in the section labelled DD. Since each of the given answers is greater than 360°360\degree and less than 720°720\degree, the arrow must have spun through more than one and less than two complete rotations. On the second spin around, the arrow will stop in the section labelled DD if the angle of rotation is greater than 216°+360°=576°216\degree+360\degree=576\degree and less than 288°+360°=648°288\degree+360\degree=648\degree. Only one of the given answers lies in this range, and so the angle of rotation must be 630°630\degree.

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