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John can build a wall in 30 days and Peter can demolish the same wall

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John can build a wall in 30 days and Peter can demolish the same wall  [#permalink]

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New post 28 Jan 2019, 03:12
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Re: John can build a wall in 30 days and Peter can demolish the same wall  [#permalink]

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New post 28 Jan 2019, 03:54
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Bunuel wrote:
John can build a wall in 30 days and Peter can demolish the same wall in 40 days.If they work on alternate days with John starting the job on the 1st day , then in how many days will the wall be built for the first time?

A. 232
B. 233
C. 234
D. 239
E. 240


John does (1/30)th of the work in a day. Let's find the number of days it takes to do (29/30)th of the work

John and Peter do 1/30 - 1/40 = 1/120 of the work in 2 days

So they do (29/30)th of the work in \([\frac{(29/30)}{(1/120)}] * 2 = 232\)days

John will do the last (1/30)th of the work in 1 day to give 233 days

Answer (B)
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Re: John can build a wall in 30 days and Peter can demolish the same wall  [#permalink]

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New post 28 Jan 2019, 03:38
Bunuel wrote:
John can build a wall in 30 days and Peter can demolish the same wall in 40 days.If they work on alternate days with John starting the job on the 1st day , then in how many days will the wall be built for the first time?

A. 232
B. 233
C. 234
D. 239
E. 240


IMO E

LCM of 30 and 40 = 120

Rate of John = 4 unit /day

Rate of Peter = 3 unit/day (demolish will be -)
D1---D2----D3-----D4
4----(-3)-----4----(-3)

Work done will be 1 unit in 2 days
120 units will be done in 240 days
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Re: John can build a wall in 30 days and Peter can demolish the same wall  [#permalink]

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New post 28 Jan 2019, 03:48
Work done by John in 1 day = \(\frac{1}{30}\)

Work done by Peter in 1 day = \(\frac{-1}{40}\)

Work done by them in 2 days is \(\frac{1}{30} - \frac{1}{40}\) = \(\frac{1}{120}\)

Work done in 1 day is \(\frac{1}{240}\)

So it takes 240 days to build the wall for first time.

OPTION: E
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Re: John can build a wall in 30 days and Peter can demolish the same wall  [#permalink]

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New post 28 Jan 2019, 05:30
1
Bunuel wrote:
John can build a wall in 30 days and Peter can demolish the same wall in 40 days.If they work on alternate days with John starting the job on the 1st day , then in how many days will the wall be built for the first time?

A. 232
B. 233
C. 234
D. 239
E. 240



rate of John = 1/30
rate of Peter = 1/40
combined rate = 1/30-1/40 = 1/120 ; 2 days work

John rate for rest of the days would be (29/30/ 1/120 ) * 2 = 232
since John has been working since day 1 ; 232+ 1; 233 days
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John can build a wall in 30 days and Peter can demolish the same wall  [#permalink]

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New post 23 Mar 2019, 05:22
VeritasKarishma wrote:
Bunuel wrote:
John can build a wall in 30 days and Peter can demolish the same wall in 40 days.If they work on alternate days with John starting the job on the 1st day , then in how many days will the wall be built for the first time?

A. 232
B. 233
C. 234
D. 239
E. 240


John does (1/30)th of the work in a day. Let's find the number of days it takes to do (29/30)th of the work

John and Peter do 1/30 - 1/40 = 1/120 of the work in 2 days

So they do (29/30)th of the work in \([\frac{(29/30)}{(1/120)}] * 2 = 232\)days

John will do the last (1/30)th of the work in 1 day to give 233 days

Answer (B)


Hi VeritasKarishma,
Why do we have to assume that john will finish the work? Why can't peter finish the work?

Below is my analysis, please can you confirm if the same is correct or not?

I guess since Peter Demolishes then John would actually have to build a little bit more than the whole wall , for Peter to finish the wall.

So if Peter were to finish then , we would actually have built more than the whole wall, and the time taken would be 240 days.
But we are asked " When was the whole wall built for the first time" we have to let John Complete the wall.
So in the last day John has to do 1/30 of the work.
Hence we have to find how much time Both John and Peter will take to do 29/30 of the work, leaving 1/30 work for John to finish up.


Hence total days is time taken by John and Peter to do 29/30 of the work plus one day that John will take to finish.

Is this analysis correct?
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John can build a wall in 30 days and Peter can demolish the same wall  [#permalink]

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New post 23 Mar 2019, 22:01
We need to assume John will work one more day since the question asks when the wall will FIRST be built.

The answer choices do not contain fractions either.

Let john work x +1 days. Let Peter work x days. John's rate is 1/30 of the wall per day. Peter's rate is -1/40 of the wall per day.

(x+1)/(30)-(x)/(40)=1

Solving for x we obtain 116. So Peter worked for 116 days and John worked for 117 days. 116+117=233 days.

Method 2:

Let the wall represent 120 units of work. Then John's rate is 4 units per day and Peter's is -3 units per day.

In two days 1 unit is completed. In 230 days, 115 units are completed.

In day 231 John produces 4 units so we have 119 units.

In day 232 Peter demolishes 3 units so we have 116 units.

In day 233 John produces 4 units so we have 120 units and have completed the job.
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John can build a wall in 30 days and Peter can demolish the same wall   [#permalink] 23 Mar 2019, 22:01
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