Dice and Division

Player A A and player B B , are playing a game. They each have an unbiased 10-sided dice with faces labelled with the numbers 1 , 2 , 3 , . . . , 10 1,2,3,...,10 . To play, A A and B B simultaneously roll their dice. The number that A A rolls is denoted as m m and the number that B B rolls is denoted as n n . If m = n m = n then they roll the dice again (ie. it is like the last roll didn't happen). If m n |m-n| divides m + n m+n then B B wins, else A A wins. The probability that B B wins can be expressed as p q \dfrac{p}{q} where p p and q q are coprime positive integers. What is p + q p+q ?


The answer is 23.

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2 solutions

Rajen Kapur
Apr 17, 2014

There is this laborious method. Draw a 10x10 table. Ruling out 10 cases of equal throws, out of 90 cases there are 48 cases where B wins. 48/90 is 8/15, i.e. answer 8 + 15 = 23.

don't we have any other method?

balaji sundaram - 7 years, 1 month ago
Kevin Bourrillion
Apr 26, 2014

There are 45 possible rolls: 9 with difference 1, 8 with difference 2, and so on through 1 with difference 9.

  • If the difference is 1: all 9 win for B, because all sums are divisible by 1.
  • If the difference is 2: all 8 win for B; odd+odd and even+even are both always even.
  • If the difference is 3: only 2 win for B; for any prime 3 or higher B only wins when both rolls are multiples of that prime
  • If the difference is 4: the 3 where both are even win for B ((2, 6), (4, 8), (6, 10)).
  • If the difference is 5: only 1, following the prime rule (5, 10).
  • If the difference is 6: 1 (3, 9).
  • If the difference is 7: none by prime rule.
  • If the difference is 8: none
  • If the difference is 9: none

9 + 8 + 2 + 3 + 1 + 1 = 24, and 24/45 = 8/15, so the answer is 23.

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