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?

a) 180 days b) 90 days c) 120 days d) 60 days

Answer is : C

Solution :

Given :-
1. A build in                 => 30 days
2. B demolish in           => 60 days

What To Find:- 
How much time will it take to build the wall if A and B work in Alternative days?

Procedure :-
1. in 1 day A can build = 1/30 th part
2. in 1 day B can be fill = -1/60 th part

So,
for first 2 days A and B can build   = 1/30 - 1/60   = 1/60
=> In 2 minutes A+B+C can  be  build   = 1/60th Part.
=> Time taken to build   1/60 part          =  2 days
=> Time taken to build   full part             =  2 * 60 days = 120 days.


(Note :- in Problem it is clear that  first 
      A will work for  1 day and then
      B will work for  1 day
      and again A and B and so on......
So for first 2 days A and B will work each for one day)







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