A can build a wall in 30 days. B can demolish that wall in 60 days. If A and B work on alternate days, when will the wall be completed?

• 117days..
  A's work in a day(1st day) = 1/30
  B's work in a day(2nd day)= -1/60(demolishing)
  hence in 2 days, combined work= 1/30 - 1/60
  =1/60
  since both works alternatively, A will work in odd days and B will work in even days.
  1/60 unit work is done in 2 days
  58/60 unit work will be done in 2*58 days
  =116 days
  remaining work=1-58/60
  =2/60
  =1/30
  next day, it will be A's turn and A will finish the remaining 1/30 work.
  hence total days=116+1=117
  
• 10 years agoHelpfull: Yes(67) No(7)
• A's 1 day work=1/30
  B's 1 day work=1/60
  Suppose A work for x days then it is clear that B work x-1 days.
  So (x/30)-(x-1)/60=1
  x=59 days (A's work)
  x-1=58 days(B's work)
  Thus wall will be completed in 59+58=117 days
  
  
• 10 years agoHelpfull: Yes(22) No(4)
• A..1day work=1/30(building the wall)
  B..1day work=1/60 (demonish the wall)
  As,they work in alternate days..
  So,in 2 days work=1/30-1/60=1/60
  So,1/60 works in 2 days
  1 work in 2*60days=120days..ans
• 10 years agoHelpfull: Yes(8) No(18)
• here it will take 120 days as because
  the net work done in 2days is (1/30-1/60)=1/60 so it will take 2*60=120 days
• 10 years agoHelpfull: Yes(6) No(7)
• For completing the wall, A will work one day more than B.
  Let B work for n days. Then A will have to work for (n+1) days to finally complete the wall.
  A's 1 day work=1/30
  B's 1 day work=1/60
  Thus,
  (n+1)*1/30 - n*1/60 = 1
  After solving, n/60 = 29/30
  n = 58
  A works for (n+1) = 59 days
  B works for n = 58 days
  Thus, Total time = 58+59 = 117 days
• 9 years agoHelpfull: Yes(3) No(2)
• The % of working of A is 100/30=3.33
  The % of working of B is 100/60=1.66
  In 2 days=(3.33-1.66)=1.66
  For complete the 100 % work=100/(3.33-1.66)=100/1.66=60.24*2=120 days.
• 9 years agoHelpfull: Yes(1) No(1)
• 59 days
  not 60 days
  
• 9 years agoHelpfull: Yes(1) No(2)
• the rate of building is double of demolishing.
  
  day 1 work = 1
  2 work = 1/2
  3 work = 3/2
  4 work = 1
  30*4= 120
• 9 years agoHelpfull: Yes(1) No(3)
• ans=
  1 day work of A is 100/30=3.33
  2 day work of A is 6.66
  1 day work of B IS 100/60= 1.75
  2 day wok is 3.5
  therefore 2 day work of (a+b)=6.66-3.5=3 approximately
  1 day wok combined 1.5
  therefore work to be completed in 100/1.5= 75 days ans
  
• 10 years agoHelpfull: Yes(0) No(10)
• 20 days :-in 20 days it will be completed
• 8 years agoHelpfull: Yes(0) No(1)
• there is a trick to solve this type of ques.
  as A is building and B is destroying.
  at first take LCM(30,60)= 60
  then divide 60 by 30 and 60... then for A=2 and B=1
  on 2nd day,, (2-1)=1
  in 60(LCM) days,, 60*2= 120 is the ans.
• 8 years agoHelpfull: Yes(0) No(3)
• correct answer is 120 if u have doubt i will make above..
• 8 years agoHelpfull: Yes(0) No(2)
• A's one day work=1/30
  B's one day work=1/60
  in 2 days , work done is =1/30-1/60= 1/60
  in 4 days , work done is =2*(1/60)=1/30
  on 5th day A will work 1/30 and complete the wall
  so ans is 5 days.
• 7 years agoHelpfull: Yes(0) No(1)