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To get to a friend’s home A has to take 2 right turns. If there are 66 possible...

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lilian-0716 | Student | (Level 1) Salutatorian

Posted August 24, 2013 at 8:39 AM via web

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To get to a friend’s home A has to take 2 right turns. If there are 66 possible routes, and using permutations, determine algebraically how many blocks downwards he would have to travel. A can only move right or down, he cannot move up or left.

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embizze | High School Teacher | (Level 1) Educator Emeritus

Posted August 24, 2013 at 4:35 PM (Answer #1)

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The paths can be described by "words" formed from the letters D(for down) and R(for right.) We are asked to determine the number of D's if the total number of permutations is 66 and the number of right turns is 2.

Let x represent the total number of letters in the "words". (E.g. if x=3 then we can have RRD,RDR,DRR as the only possible paths since there are 2 R's with R=right turn and D=down turn.)

Then the number of permutations, 66 , can be represented as:

`(x!)/((x-2)!2!)=66` (x! represents the total number of words -- we divide to find the number of distinguishable permutations.)

`(x!)/((x-2)!2!)=66`

`(x(x-1)(x-2)!)/((x-2)!2)=66`

`x(x-1)=132`

`x^2-x-132=0`

`(x-12)(x+11)=0`

`x=12,x=-11` The second answer does not make sense in context, so x=12.

Thus the length of the words is 12; there are 2 R's so there must be 10 D's.

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A must take 10 down turns

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