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# If Johnny can mow the lawn in 30 minutes and with the help of his brot

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Math Expert
Joined: 02 Sep 2009
Posts: 62496
If Johnny can mow the lawn in 30 minutes and with the help of his brot  [#permalink]

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17 Feb 2020, 07:34
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Difficulty:

15% (low)

Question Stats:

96% (01:23) correct 4% (01:51) wrong based on 24 sessions

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If Johnny can mow the lawn in 30 minutes and with the help of his brother, Bobby, they can mow the lawn 20 minutes, how long would it take Bobby working alone to mow the lawn?

(A) 1/2 hour

(B) 3/4 hour

(C) 1 hour

(D) 3/2 hours

(E) 2 hours

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Joined: 12 Oct 2015
Posts: 232
GMAT 1: 700 Q47 V39
GPA: 3
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Re: If Johnny can mow the lawn in 30 minutes and with the help of his brot  [#permalink]

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17 Feb 2020, 14:37
1

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Joined: 22 Sep 2014
Posts: 157
Location: United States (CA)
Re: If Johnny can mow the lawn in 30 minutes and with the help of his brot  [#permalink]

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17 Feb 2020, 15:32
1
John : 30 min
John + Bobby : 20 min

J30=j20+b20
J10=b20

So we can get that it takes Bobby 20 min to do what John can do in 10 min

Since it takes John 30 min to finish the whole thing , it will take Bobby 60 min

C

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Intern
Joined: 01 Oct 2016
Posts: 2
Re: If Johnny can mow the lawn in 30 minutes and with the help of his brot  [#permalink]

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17 Feb 2020, 15:51
Assuming total work equals to 1
Rate = work/time
Rate(johny)=1/(30/60)=2
Rate(j)*20/60 + rate(b)*20/60 =1
Implies rate(b) =1
Therefore to complete work alone bobby would have to eork one hour

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Joined: 18 Aug 2017
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Location: India
Concentration: Sustainability, Marketing
GPA: 4
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Re: If Johnny can mow the lawn in 30 minutes and with the help of his brot  [#permalink]

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18 Feb 2020, 01:45
Bunuel wrote:
If Johnny can mow the lawn in 30 minutes and with the help of his brother, Bobby, they can mow the lawn 20 minutes, how long would it take Bobby working alone to mow the lawn?

(A) 1/2 hour

(B) 3/4 hour

(C) 1 hour

(D) 3/2 hours

(E) 2 hours

given
1/30=1/x+1/20
x= 60
; 1 hr
IMO C
Re: If Johnny can mow the lawn in 30 minutes and with the help of his brot   [#permalink] 18 Feb 2020, 01:45
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