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Vandaag is het 26 September 2023

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Keep the money rolling - math puzzle


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You put two 1 dollar coins on a table, such that they touch each other (see the figure). While you fix the lower coin with your finger, you will turn the upper around it (without slipping). Somewhat later the moving coin will be back in his original position. How many rotations on his midpoint did the moving coin make? coin


In fig. 1 the coins touch each other one the places marked with a cross. If the upper coin has rotated a quarter of his circumference around the lower one, the tocuh each other at the two tiny circles. (see figure 2). The moving coin is then upsite down. So he made half a rotation. So in order to complete the whole tour the coin makes 2 rotations. euro