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jmc

counting and probability senior

Problem

On the game board below Kendra will start at the center of the board. For each turn she will spin this spinner with four congruent sectors once, and then she will move one space in the direction indicated on the spinner. The "Start'' square does not have a numerical value, but Kendra may land on it during her turns. What is the probability that the sum of the numbers in the spaces on which she will land will be exactly 30 after her third complete turn? Express your answer as a common fraction.

problem


problem
Solution
On her first turn, Kendra must get 10 points. If she wants a total of 30 after three turns, she must then either get two tens in a row or a 5 and then a 15. To get three tens in a row, she can move any direction on her first move, go in two possible directions for her second move, and go in two possible directions for her third spin, meaning her probability of success is . On the other hand, if she wants to get a 10, a 5, and a 15, she can only move left or right on her first move, further out on her second move, and then up or down on her third move, leading to a probability of success of . Adding, her total probability is .
Final answer
\frac{5}{16}