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Bunuel
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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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Bunuel
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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