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18 Jun 2013, 22:16
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C
Be sure to select an answer first to save it in the Error Log before revealing the correct answer (OA)!
Difficulty:
75%
(hard)
Question Stats:
36% (02:48) correct
64%
(02:15)
wrong
based on 11
sessions
History
Date
Time
Result
Not Attempted Yet
A runner starts running on a circular path of radius 'r' meters. His average speed (in meters/min) is ∏r during the first 30 sec, ∏r/2 during the next one minute, ∏r/4 during the next 2 min, ∏r/8 during the next 4 min and so on... What is the ratio of the nth round to the previous round?
A) 4
B) 8
C) 16
D) 32
E) 64
A) 4
B) 8
C) 16
D) 32
E) 64
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19 Jun 2013, 22:20
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atalpanditgmat
I believe the last sentence should say: What is the ratio of the time taken for the nth round to that for the previous round? Without stating time, the question can't really be answered.
Given that information, though:
Time = Distance / Velocity
In the first interval: Velocity = (∏r meters/min) => (∏r/2 meters in 30 seconds) => Quarter-Lap in 30 seconds: T = 30 seconds, D = Quarter-Lap, Velocity = ∏r
Since the velocity is being halved and the time is being doubled, the distance is clearly constant: D = V * T = (V_old * 0.5) * (T_old * 2) = D_old
So at each velocity and time shift, we are covering a quarter-round of distance. We need to compare the times to complete 4-interval rounds.
Since T = D/V, and D is constant, T will vary inversely with V.
Consider how the velocity for a quarter round in one cycle will change with respect to a quarter round in the next cycle:
Since the velocity is halved each time, it will be sixteenth-ed after each series of 4 quarter-rounds. (1/2 * 1/2 * 1/2 * 1/2 = 1/16).
So since each round's velocity is 1/16 of the previous round's velocity, the time is multiplied by 16.
So, -C-.
ConnectTheDots
20 Jun 2013, 01:10
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atalpanditgmat
The problem is faulty. The sequence (∏r during the first 30 sec, ∏r/2 during the next one minute, ∏r/4 during the next 2 min, ∏r/8 during the next 4 min and so on...) is an infinite GP series. If the person keeps on taking double the time for next half distance, he can never reach the end point to complete 1 round.
Distance series , ∏r + ∏r/2+ ∏r/4+ ∏r/8+ ∏r/16+ ∏r/32+ .... +∏r/2^(n-1) = 2∏r
∏r(1- 0.5^n)/(1-0.5) = 2∏r
1-0.5^n = 1
0.5^n = 0
n = infinite
So the time series for round 1: 30 + 60 + ... infinite times is undefined.
what is the source of this problem ?
He will keep on reducing the speed and finally in an infinite time he will complete 1 circle.
