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CAN YOU PLEASE TELL ME ...HOW DO THEY DO IT????

SOMEBODY SOLVE ME THIS ANSWER.
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  • Assume 10 players numbering 1 to 10.
    no 1 shake hands with all other 9,
    no 2 with all remaining 8 bcoz he already did with 1
    likewise 3 with 7, 4 with 6....... 9th man with 1

    9+8+7....+1=>Its an arithmetic progression n(n+1)/2, where n=9=>45
  • This is a simple n(n-1)/2 problem or 10C2 problem.

    You can answer this 10*9/2 = 45

    This comes naturally to anyone who has done any aptitude paper, NTSE, CAT, MAT , PO or any government examination or sat for placement in his college.

    So if you are seeing this question for the first time in UPSC Prelims, then you are not preparing adequately.

    Now solving this problem. You can use perm and combination.

    Out of 10 people, you have to choose pairs of 2 such that no two pairs are repeated.

    You can do a 10C2 = 10*9/2

    But if you known nothing at all, you can solve this problem by induction.

    Imagine with a smaller set 3 people, how many handshakes?

    P1, P2, P3

    (P1-P2) (P1-P3) = 2 handshakes
    (P2-P3) = 1 Handshakes

    Total (2+1 ) = 3 handshakes



    Now do it with 5 people

    P1,P2,P3,P4,P5

    (P1-P2),(P1-P3),(P1-4),(P1-P5) = 4

    (P2-P3),(P2-P4),(P4-P5) = 3

    (P3-P4),(P-5) = 2

    (P4-P5) = 1

    TOTAL = 4+3+2+1=10

    Now think of 10 people

    You can see the Math = 9+8+7+...+1 = Sum of numbers till one less than total number of people

    Sum of numbers till (10-1) = 9

    = n(n+1)/2 = 9*10/2 = 45

    Hope this helps.
    *No good deed goes unpunished*
  • edited January 2017

    SOMEBODY SOLVE ME THIS ANSWER.

    2 players r reqd for a handshake..pick any 2 out of 10...so combinatory formula is 10C2 ( 10!/2! 8! ) = (90/2) = 45
  • SOMEBODY SOLVE ME THIS ANSWER.

    You can solve it as above. Although for aptitude Tests, it is recommended that you solve a few hundred questions so that none of the questions are new to you.

    Especially such questions. They are very very common.
    *No good deed goes unpunished*
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