One man builds a wall in 21 months and other one destroys it in 28 months. If they work alternate than in how many months wall will complete?

• A=100/21=4.761,B=100/28=3.5718(-ve work)
  total=4.761-3.5718=1.19047
  time to complete 96% of work=96/1.19047=80month due to altenate work total month=2*80=160
  now for next one month the work get completed
  so total month of work=160+1=161
• 10 years agoHelpfull: Yes(10) No(1)
• This type of questions are very easy to solve guys. So lets do it in a easy way!!!
  
  For our ease, we will think this ques like a story of 2 persons eating and ignoring chocolates.
  
  Take LCM of 21 and 28. It is 84, i.e. 84 chocolates are there for A and B. So the work is to finish these 84 chocolates. A will eat and B will ignore chocolates.
  
  Divide 84 from 21 and 28, i.e, 4 and 3.
  
  Now,our ques becomes very easy to solve.
  
  A will eat 4 chocolates in a day and B will throw(destroy) 3 chocolates in a day.
  
  Acc to ques,
  They will work in alternate days,
  2nd day work -----> (4-3=1 chocolate)
  .
  .
  .
  160th day work --------> 80 chocolates.
  Now 4 chocolates left and A will start the work acc to ques.
  So he will eat remaining 4 chocolates in 1 day.
  
  Therefore, 160+1= 161 days
• 8 years agoHelpfull: Yes(10) No(0)
• 1 month work of wall making man=w/21
  1 month work of wall destroying man =w/28
  total work done in 2 month to make wall=(w/21-w/28)=w/84
  so,w/84 wall made in=2 month
  w work to complete wall=2*84*w/w=168 month
  but we know that they are working alternatively.so even days destroying man is working.
  it means wall have been already made.
  it means thay both worked for 160 month together and then in 161 month wall made
  so answer is 161 month
• 10 years agoHelpfull: Yes(6) No(1)
• 161/7=no remainder i.e. 161
• 10 years agoHelpfull: Yes(2) No(1)
• 168 days
• 10 years agoHelpfull: Yes(1) No(7)
• 161 is the ans
• 9 years agoHelpfull: Yes(1) No(0)
• 161 is the correct answer
• 8 years agoHelpfull: Yes(0) No(0)