A circular spinner is divided into five equal sections. An arrow is attached to the centre of the spinner and is positioned as shown.
The arrow is spun clockwise, and it stops in the section labelled D. Which of the following could have been the angle of rotation?
Pick one
Solution
The angle of one complete rotation measures 360°. Since the spinner is divided into 5 equal sections, each 5360°=72° rotation moves the arrow to the next dividing line between two sections. Rotating clockwise, the section labelled D is the 4th section. This means that if the angle of rotation is greater than 3×72°=216° and less than4×72°=288°, then the arrow stops in the section labelled D. Since each of the given answers is greater than 360° and less than 720°, 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 D if the angle of rotation is greater than 216°+360°=576° and less than 288°+360°=648°. Only one of the given answers lies in this range, and so the angle of rotation must be 630°.
Want a route through all this instead of an archive? The track
puts 2,604 problems in a working order, from Junior Challenge level to the IMO shortlist.