Latest Gate Aptitude Question SOLUTION: A and B are working on a job.A is the builder and B is the demolition man.A takes 20 days to construct a wall completely.B takes 40 days to demolish it completely.How much time do

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(#M40013703) GATE question Keep an EYE

A and B are working on a job.A is the builder and B is the demolition man.A takes 20 days to construct a wall completely.B takes 40 days to demolish it completely.How much time do they take to build the wall completely if they work alternatively.If A starts the work

Asked In Gate venkatesh bollam (11 years ago)

Solved Vinay Sipani Read Solution (4)

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Read Solution (4) :

Vinay Sipani   (11 years ago)

A builds 1/20 th wall in one day
B destroys 1/40 th wall in one day

=> In two days, (1/20) - (1/40) = (1/40)th wall is built
=> Average wall built in one day = 1/80

=> In 78 days , (78/80) th wall is built
On 79th day, a takes 1/2 day to build remaining 2/80 th wall

=> Total time taken = 78.5 days

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Amit kumar  (11 years ago)

Total work = LCM(20,40)=40 units
For A
20 days works = 40 units
1 day works = 40/20=2 units
For B
40 days work = 40 units
1 day work = 40/40 = 1 unit
If A start the work and they do alternately.
in First 2 day (work done by A and B) = 2-1= 1 unit
2*38=76 days = 1*38 = 38 units work
76 days + 1 day(last day A works)= 38 units+ 2 units
total 77 days = 40 units work(which is total work)
the Total time Taken = 77 Days

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sc mandota  (11 years ago)

as they do only 1/20-1/40 work in two days=1/40
they take 76 days to complete 38/40
on 77th day= 38/40+1/20=1
so 77 days

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Milind   (8 years ago)

Ans 79
On day 1 A will build 1/20 of the wall
On Day 2 B will break 1/40th of it. Thus at the end of each 2 day period 1/40th of the wall will be built.
Hence on the 78th day 39/40th of the wall will be built.
On the 79th Day A will complete the wall fully

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