A secret can be told only 2 persons in 5 minutes .the same person tells to 2 more persons and so on . How long will take to tell it to 768 persons ?

• only one person is telling the secret.
  2 person knows it in 5 mins
  another 2 i.e. 4 persons knows in 5 more mins i.e. 10 mins and so on.
  thus 768 person knows the secret in (768/2)*5 mins = 1920 mins.
• 13 years agoHelpfull: Yes(1) No(6)
• Ans is 50 min.
  1->2->4->8->16......->512->1024
  2^9=512 After this definitely one person need 5 min to tell to others. So totally 10 * 5 min =50 min
• 13 years agoHelpfull: Yes(1) No(6)
• All options are incorrect. Reason:
  
  I can interpret the above question in four ways:
  
  1. A person can tell ONLY to 2 other person in 5 mints, and then he can't tell anyone any-more. Then other 2 people tell more people and so on.
  
  2. A person can tell to 2 new person in 5 mints and then he continues telling other people along with new 2 people and so on.
  
  3. Assuming the first case true, a person tell two people in 5 mints serially, i.e 2.5 mint for each person.
  
  4. Assuming the 2nd case true, a person tells two people in 5 mints serially, i.e 2.5 mint for each person.
  
  Solution to 1st interpretation:
  
  0th mint 1 person knows
  5th mint 2 new person know (total 3)
  10th mint 4 new person know (total 7)
  15th mint 8 new person know (total 15)
  - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
  40th mint 256 new person know (total 511)
  45th mint 512 new person know (total 1023)
  
  Now if I exclude the first person, in 40 mints 510 new person were told, and in 45th mint 1022 new person were told, so the answer is 45.
  
  
  Solution to 2nd interpretation:
  
  0th mint 1 person knows.
  5th mint 2 new person know. (total 3)
  10th mint 6 new person know.(total 9)
  15th mint 18 new person know. (total 27)
  20th mint 54 new person know. (total 81)
  25th mint 162 new person know. (total 243)
  30th mint 486 new person know. (total 729)
  35th mint 1558 new person know.. (total 2187)
  
  Now if I exclude the first person, in 30 mints 728 new person were told, and in 35th mint 2186 new person were told so the answer in 35.
  
  
  
  Solution to 3rd interpretation:
  
  0th mint 1 person knows.
  2.5th mint 1 new person knows (total 2)
  5th mint 2 new person know (total 4)
  7.5th mint 3 new person know (total 7)
  10th mint 5 new person know (total 12)
  ------------ - - - - - - - - - - - - - note that the number of new person follows Fibonacci series.
  
  1 1 2 3 5 8 13 21 34 55 89 144 233 377
  
  Each 2.5 mint new numbers added. If we move upto 13th term total is 509.
  If we move upto 14th term the total is 846. So to upto 14th term we need 13*2.5 mints, 32.5 mints
  
  
  
  Solution to 4th interpretation:
  
  Here the problem reduces to "each person tells to one person in 2.5 mints and he never retires, i.e keeps telling new people.
  
  So the series is:
  1 1 2 4 8 16 32 64 128 256 512
  
  Each new term comes in 2.5 mints, and the new term is basically sum of all old terms, because each old person will tell a new person in 2.5 mints.
  
  So the 17.5 mint, total 512 (1+1+2+ 4+ 8+16+32+64+128+265) person will know. At 20th mint 1024 person will know. So the answer in 20 mint.
• 11 years agoHelpfull: Yes(0) No(0)