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John takes 20 min to walk every kilometer (km). David takes 16 min to

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John takes 20 min to walk every kilometer (km). David takes 16 min to  [#permalink]

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New post 25 Jun 2019, 03:15
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A
B
C
D
E

Difficulty:

  55% (hard)

Question Stats:

69% (03:04) correct 31% (03:02) wrong based on 95 sessions

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Re: John takes 20 min to walk every kilometer (km). David takes 16 min to  [#permalink]

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New post 26 Jun 2019, 03:30
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Bunuel wrote:
John takes 20 min to walk every kilometer (km). David takes 16 min to walk every km. Both of them walk in a race. If David beat John by 240 meters (m), find the length of the race (in km)? (1 km = 1000 m)

(A) 1
(B) 1.2
(C) 1.4
(D) 1.5
(E) 1.6


1000 mtrs; speed mtr per min ; 1000/20 ; 50 and 1000/16 ; 62.5

distance be x
since time duration would be same so
time taken by David to cover x mtrs; x/62.5and John x-240/50
so x/625= x-240/50
solve for x= 1200 mtrs or say 1.2 mtr
IMO B
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Re: John takes 20 min to walk every kilometer (km). David takes 16 min to  [#permalink]

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New post 25 Jun 2019, 03:54
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Bunuel wrote:
John takes 20 min to walk every kilometer (km). David takes 16 min to walk every km. Both of them walk in a race. If David beat John by 240 meters (m), find the length of the race (in km)? (1 km = 1000 m)

(A) 1
(B) 1.2
(C) 1.4
(D) 1.5
(E) 1.6


Convert to Meters john walks 3000m per hour and david 3750m per hour so david gains over 750 meters in 1 hour. david requires 240/750*60 hrs to win by 750. just less than 20 minutes. so david will cover 1.2 km in 20 min IMO B
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Re: John takes 20 min to walk every kilometer (km). David takes 16 min to  [#permalink]

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New post 25 Jun 2019, 04:13
I am not sure if my answer is correct

speed of John-3km/hr
speed of David-3.75km/hr

when time is constant

D(David)=3*t
D(John)=3.75*t
when t=1
D(J)-D(D)=0.75km

for David to beat John by 0.75km it takes 3.75 km hence to beat by for 0.24km it will take 1.2km.and that will be length of the race as David is faster and he will reach the finish point first .IMO option B
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Re: John takes 20 min to walk every kilometer (km). David takes 16 min to  [#permalink]

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New post 01 Jul 2019, 08:08
Bunuel wrote:
John takes 20 min to walk every kilometer (km). David takes 16 min to walk every km. Both of them walk in a race. If David beat John by 240 meters (m), find the length of the race (in km)? (1 km = 1000 m)

(A) 1
(B) 1.2
(C) 1.4
(D) 1.5
(E) 1.6



Main Formula : D=RT
JOHN :
D = D-0.2 (240/1000)= 0.2
R = R
T = 20

D = RT
D-0.2 = 20R
D-0.2/20 = R --- 1)

Likewise for David
D=RT
D=D
R=R
T=16
Or , D = R16
Or, D = 16(D-0.2)/20 [From Equation 1]
Or D ~ 0.7

For JOHN D = 0.7 - 0.2 = 0.5

Total D = 0.7 + 0.5 = 1.2 Answer B
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Re: John takes 20 min to walk every kilometer (km). David takes 16 min to  [#permalink]

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New post 29 Jul 2019, 07:54
1
john=50m/min
david=62.5m/min
David beats john by 240 meters
Realtive speed=12.5meters/min
time taken to complete 240 meters=240/12.5 -->19.2 minutes(overall time of the race)
David walked for 19.2 minutes--> 19.2*62.5=1200 meters-->1.2km
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Re: John takes 20 min to walk every kilometer (km). David takes 16 min to  [#permalink]

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New post 02 Aug 2019, 00:48
BASIS OF SOLUTION:
The ratio of the speeds of two moving objects can be expressed in two ways:
(a) As the ratio of the respective distances they cover in any given time (e.g., if S1 and S2 are the speeds and D1 and D2 are the distances covered in any given time, then S1/S2=D1/D2)
(b) As the inverse of the ratio of the time taken by each to cover any given distance (again, if S1 and S2 are the speeds and T1 and T2 are the times taken to cover any given distance, S1/S2=T2/T1)

SOLUTION:
(a) Let X meters be the length of the race. In the time that David takes to cover X meters, John covers (X-240) meters. If SD and SJ are David's and John's speeds then:
SD/SJ=X/(X-240)
(b) David and John cover 1 km in 16 minutes and 20 minutes respectively. Therefore:
SD/SJ=20/16
Thus, X/(X-240)=20/16
X=1200 meters=1.2 kms. ANS: B
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Re: John takes 20 min to walk every kilometer (km). David takes 16 min to   [#permalink] 02 Aug 2019, 00:48
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John takes 20 min to walk every kilometer (km). David takes 16 min to

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