Latest CTS Aptitude Question SOLUTION: a secret can be told only 2 persons in 5 mins.the same person tells to 2 more persons and so on.how long will it take to tell 768 people. a)47.5min b)50min c)500min d)49min

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a secret can be told only 2 persons in 5 mins.the same person tells to 2 more persons and so on.how long will it take to tell 768 people.
a)47.5min
b)50min
c)500min
d)49min

Asked In CTS Shruti Ghosh (12 years ago)

Unsolved Read Solution (11)

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Read Solution (11) :

Sibin  (12 years ago)

5 minute we can told to 2 person.For 2.5 minute we can talk to one person.

2-4-8-16-32-->2^2-2^3-2^4 etc....
2^9 means 512 .
so 9*5 =45 minute
half of the 512 is 256
512+256 = 768
9*5+2.5 =45+2.5=47.5 minute

Is this solution Helpfull? Yes (17) | No (6)  

Dolly  (12 years ago)

None of these
as
correct ans will be
32.5 mins

First person will tell secret to 2 persons in 5 mins.
After 5 mins, 3 persons will know about the secret.

Then in next 5 mins ( total 10 mins) .... 6 additional persons know the secret.
Means total 9 persons know the secret after 10 mins.

so chain will be
1,3,9,27,81,243,729

after 30 mins, 729 persons know the secret.
In next 2.5 mins, 729 more will know the secret.

so after 32.5 mins, 768 persons will know the secret.

Is this solution Helpfull? Yes (13) | No (7)  

Mithlesh Prasad  (12 years ago)

1 person will tell 2 persons.
those two will tell 4 persons.
those 4 will tell 8 persons.
and so on,16,32,64,128,256 and those 256 people will tell another 256 people in 2.5 min
so,adding time and people we get 768 people in 47.5 min.
so option a will be correct choice.

Is this solution Helpfull? Yes (4) | No (1)  

Vignesh  (12 years ago)

None of these options are correct.
Agree with Dolly.
Around 31min take to tell 768 people..

Is this solution Helpfull? Yes (3) | No (2)  

Vasu  (12 years ago)

Dolly is definitely right.
Time taken will be 32.5 mins
as one person will tell secret to other in 2.5 mins.

well explained by Dolly.

Is this solution Helpfull? Yes (3) | No (4)  

sruthi  (10 years ago)

5 min-2
10-4
15-8
20-16
25-32
30-64
35-128
40-256
45-512
47.5-768

so 47.5 min is the answer

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sucharitha  (12 years ago)

after 2.5 minutes more 729 will know the secret that means after 32.5 minutes 1458 people will know the secret that means more than 768 person knows the secret.we cant tell how much time it will take exactly

Is this solution Helpfull? Yes (2) | No   

Jay Kumar Jain  (11 years ago)

no dolly u dnt assume d first person d secret is being told to 2 persons simulataneously and u dnt count if dere is a actual person telling dem d secret

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abhirami  (12 years ago)

2+4+8+16+....=should reach 768
therefore sum(geometric series)=2(2^n-1)(2-1)should exceed 768
on solving, i got n=9.588 therefore chose n=10 levels.therefore total time taken=10x5=50.

Is this solution Helpfull? Yes (1) | No (4)  

vikash kumar  (12 years ago)

i am agree with dolly.........because the chain will 1,3,9,27,81,243,729 so after 30 minutes 729 persons will know the secret.in 2.5 minute one person will tell the secret to another one,so new 729 person will know the secret in next 2.5 minute.

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Abhijit   (12 years ago)

50 min...1(5min),1+2(after 10 min),1+2+2^2(after 15 min) so 1+2+2^2+....2^n=768 where time required=(n+1)*5
solving these two we get 2^(n+1)-1=768 or n=9(to tell the secrect requires 5 min not less than that)...so answer is 50min

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