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

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Bunuel
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If Johnny can mow the lawn in 30 minutes and with the help of his brot [#permalink] New post   17 Feb 2020, 08:34
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94% (01:18) correct 6% (01:44) wrong based on 36 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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C
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jkolachi
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Re: If Johnny can mow the lawn in 30 minutes and with the help of his brot [#permalink] New post   17 Feb 2020, 15:37
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Re: If Johnny can mow the lawn in 30 minutes and with the help of his brot [#permalink] New post   17 Feb 2020, 16:32
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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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Re: If Johnny can mow the lawn in 30 minutes and with the help of his brot [#permalink] New post   17 Feb 2020, 16:51
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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Re: If Johnny can mow the lawn in 30 minutes and with the help of his brot [#permalink] New post   18 Feb 2020, 02:45
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
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gmatclubot
Re: If Johnny can mow the lawn in 30 minutes and with the help of his brot [#permalink]
18 Feb 2020, 02:45
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