Maths Olympiad Prep

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Combinatorics Difficulty 2.5 Junior Find the answer Canada

In the game shown, Riah begins on the start square. She rolls two
standard six-sided dice and moves forward the number of squares equal to
the sum rolled. She continues in this way, rolling two dice and moving
forward until she lands on or passes the finish square. If she lands on
a shaded square, she follows the instructions shown. If Riah begins with
00 points, what is the greatest
number of points that she can finish this game with?

IMG0

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Solution

When two standard six-sided dice are rolled, the minimum sum that
can be rolled is 1+1=21+1=2, and so Riah must move forward at least 22 squares on each roll. This means that Riah cannot land on the square numbered 11, she can land on at most one of the squares numbered 1111 and 1212, and she can land on at most one of the squares numbered 1616 and 1717. To achieve the maximum score possible, the goal is to land on as many shaded squares as possible, recognizing that a choice must be made between squares 1111 and 1212, and also between squares 1616 and 1717. To begin the game, Riah cannot roll a 11, but she can roll a 33, thus adding 66 points to her score. She can then roll 33 again, adding 44 points to her score, followed by a roll of 22 to add 55 more points to her score. Riah is now on the square numbered 88, she has 6+4+5=156+4+5=15 points, and she has collected the maximum number of points possible to this point in the game. From the square numbered 88, Riah can roll a 33 and collect 77 points or she can roll a 44 and collect 88 points. Since landing on square 1111 or 1212 does not affect her ability to land on square 1616 or 1717, Riah maximizes her points by rolling 44 and collecting 88 points. She is now on the square numbered 1212 and has 15+8=2315+8=23 points. From the square numbered 1212, Riah can roll a 44 and collect 1010 points or she can roll a 55 and double her points. Since adding 1010 points gives Riah 23+10=3323+10=33 and doubling her points gives 23×2=4623\times2=46, then rolling a 55 and landing on the square numbered 1717 maximizes Riah's points. There are no more points that can be collected before finishing the game. Thus, the greatest number of points that Riah can finish the game with is 4646.

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Source: CEMC, University of Waterloo, licensed CC-BY-NC-4.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project.