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John can build a wall in 30 days and Peter can demolish the same wall
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Updated on: 27 Aug 2023, 03:23
Updated on: 27 Aug 2023, 03:23
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18% (01:42) correct
81% (01:59) wrong based on 22 sessions
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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?
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233
Originally posted by Sarasai019 on 26 Sep 2018, 06:01.
Last edited by Carcass on 27 Aug 2023, 03:23, edited 1 time in total.
Last edited by Carcass on 27 Aug 2023, 03:23, edited 1 time in total.
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Re: John can build a wall in 30 days and Peter can demolish the same wall
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26 Sep 2018, 13:09
26 Sep 2018, 13:09
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Sarasai019 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?
I get 240 as the answer. But certain people say its 233. Not sure about the right answer to this. Any help solving this is appreciated! Thanks
I get 240 as the answer. But certain people say its 233. Not sure about the right answer to this. Any help solving this is appreciated! Thanks
John can build a wall in 30 days and Peter can demolish the same wall in 40 days
So, in ONE day, John can build 1/30 of the entire wall
And, in ONE day, Peter can demolish 1/40 of the entire wall
So, AFTER TWO DAYS, the portion of the wall standing = 1/30 - 1/40 = 1/120
So, AFTER TWO DAYS, 1/120 of the wall is built (and remains standing)
So, after FOUR DAYS, 2/120 of the wall is built (and remains standing)
And after SIX DAYS, 3/120 of the wall is built (and remains standing)
.
.
.
And after 232 DAYS, 116/120 of the wall is built (and remains standing)
On DAY 233, it's John's turn to build.
Remember that, in ONE day, John can build 1/30 of the entire wall
In other words, in ONE day, John can build 4/120 of the entire wall
So, after 232 DAYS, 116/120 of the wall was built, and on the 233rd day, John built the remaining 4/120 of the wall.
So, WALL COMPLETE!!
Answer: 233
Cheers,
Brent
General Discussion
Re: John can build a wall in 30 days and Peter can demolish the same wall
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27 Sep 2018, 06:44
27 Sep 2018, 06:44
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Thanks much for such a detailed and clear explanation, Sir! I just learnt from internet about a technique for solving time and work problems. It actually worked out for most of the problems I solved but it was otherwise with this one problem. I'm just curious to learn where did I go wrong in either understanding or applying this concept. So, what I did using that concept is this - " Assume total work to be 120 units. So, John will do 40 units of work per day and Peter will (un)do 30 units of work per day. So, in a span of two days, 10 units of work would be done. This implies, in a span of 20 days, 100 units of work would be completed. On the 21st day, it would be John's turn and he can do 40 units/day but we want just 20 units/day, therefore, the work can be completed in 20.5 days". I am still unclear as to why this particular method flunked or how to apply the same method of assuming a value for work for this type of question! Thanks in advance!
