47.5 mins
because 2^9=512
& 2^10=1024 but
total no. of persons=768
i.e 512+256(or 2^8)
so for 9th iteration it is 9*5=45mins
but after that only half persons(512/2=256) are to be
informed which will take 2.5mins(5/2). so
the final answer will be 45+2.5=47.5 mins....

First in an aptitude question students are expected to keep
their eyes open to the terms used in the question. Otherwise
an aptitude paper would be the same as that of a 6th or 7th
standard question paper. The original question here was "the
same person". So the same person tells it to different
couple of people taking 5 mins each. 768 person here, so
768/2=384,384*5=1920 should have been the answer.

Now, supposing its a typical maths question, i.e. assuming
all the above assumptions made in the earlier posted answers
to be true, its a simple GP series which starts from 1, then
2 then 4 then 8 and so on.

So, by the problem,
2^0 + 2^1 + 2^2 +....+ 2^n = 768 (note: the first person has
been counted in the series)

so putting the summation formula for GP
1.(2^(n+1)-1) / (2-1) = 768
=> 2^(n+1) = 769
Now, 2^9 = 512 (leaving 257 behind)
Again, the count of 5 mins will be n number of times cause
the first term, i.e. 2^0 is the first person and he already
knows the secret, so the count will start essentially after
him. So n+1=9 => n=8. And for the remaining 257(actually
256, as the first person is counted in it) will be told in
extra 2.5 mins. So total, 8*5 + 2.5 = 42.5

@Crazyfantaxy
because it is told that the same person is telling that
secret to two new people..and then to two new people..and so
on..so the people who have been told will not be taken into
account...47.5 is therefore correct ans which Harry has given

till now total people knowing secret is
1+2+4+8+16+32+64+128+256=511(approxx) heee..

for next 5 min all 256 people will be telling crap secret to
their next 2 people each.so in 5 minutes
we will have 768 people.time required will be same either
we need 257 people or 512 i.e 5minutes

Take this question as challange --> "Candle 'A' takes just
four hours to burn it completely and candle 'B'
takes 2 hrs. The girth of candle 'A' is just double of
candle 'B' with the same height. If Ram lit both the
candles in the same time and after how long time the length
of Candle 'B' would be just half of candle 'A'?

The cost of four dozen proof machine ribbons and five dozen
accouting machine ribbons was Rs.160/-. If one dozen
accounting machine ribbons cost Rs.20/-, what is the cost of
a dozen proof machine ribbons?

Mr. Beans visited a magic shop and bought some magical
marbles of different colours along with other
magical items. While returning home whenever he saw a
coloured light, he took out marbles of similar
colours and counted them. So he counted the pink coloured
marbles and found that he has bought 25 of
them. Then he counted 14 green marbles and then 21 yellow
marbles. He later counted 30 purple coloured
marbles with him. But when he reached a crossing, he looked
at a red light and started counting red marbles
and found that he had bought 23 Red marbles. As soon as he
finished counting, it started raining heavily
and by the time he reached home he was drenched. After
reaching home he found that the red, green and
yellow marbles had magically changed colours and became
white, while other marbles were unchanged. It
will take 1 day to regain its colours, but he needs to give
atleast one pair of marbles to his wife now. So
how many white marbles must be choose and give to his wife
so as to ensure that there is atleast one pair of
red, yellow and green marbles ?