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6 horses {A,B,C,D,E,F} Participate in a race. If there are n

This topic has 1 expert reply and 1 member reply

Top Member

6 horses {A,B,C,D,E,F} Participate in a race. If there are n

Post Thu Sep 28, 2017 5:27 am
Elapsed Time: 00:00
  • Lap #[LAPCOUNT] ([LAPTIME])
    6 horses {A,B,C,D,E,F} Participate in a race. If there are no draws in the rate then in how many ways can the race end such that horse A always finishes ahead of C and B both the horses?

    A) 720
    B) 360
    C) 240
    D) 120
    E) 48

    Source: www.GMATinsight.com

    Answer: Option C

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    Post Thu Sep 28, 2017 7:11 am
    A finishes 1st: 5 options for B x 4 options C x 3 x 2 options for D,E,F = 120
    2nd: 4....3...." = 72
    3rd: 3.....2..." = 36
    4th: 2........." = 12

    Total 240 C

    Post Tue Oct 10, 2017 5:27 am
    Hello GMATinsight and Regor.

    I would like to give you a more detailed solution.

    We have 6 horses and we can have the following cases:

    - Horse A finishes 1st: here, we have to organize 5 horses in 5 places, the number of ways to do that is A*5*4*3*2*1=5!=120.

    - Horse A finishes 2nd: First, we have to pick the horse for the first place but we can not select horses B or C. So, we have 3 options for the first place (D, E, F) and the number of ways to select a horse for the first place is 3. Then we have to organize 4 horses in the last 4 places. The total number of ways in this case is 3*A*4*3*2*1=72.

    - Horse A finishes 3rd: as in the case before, we have to pick 2 horses for the first 2 places but we can not select horses B or C. So, we have 3 options again and we can organize this 2 horses in 3*2=6 different ways. Now we have to organize the last 3 horses in the last 3 places. The total number of ways in this case is 3*2*A*3*2*1=36.

    - Horse A finishes 4th: Now, we have to pick 3 horses for the first 3 places but we can not select horses B or C, the number of ways to organize this 3 horses is 3*2*1=6. Finally, we have to organize 2 horses (B and C) in the last 2 places. The total number of ways in this case is 3*2*1*A*2*1=12.

    You should noticed that A can not finish in the 5th place or in the 6th place.

    In conclusion, the total number of ways this race can end is 120+72+36+12=240.

    So, the correct answer is C.

    I hope this help you. I'm available if you'd like any follow up.

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