obama can build a wall in 21 months and osama can destroy in 28 months,they worked alternately .in how many days they will complete the work?
a)161b)168c)167d)163

• obama build in 21 months
  osama sala terorist and destructor 28 months
  
  take lcm of 21 and 28 =7*3*4 unit=total work
  so obama efficency =7*3*4 /21=+4 (+ for construction)
  osama(terorist) destructor =-7*3*4/28 =-3(constructor)
  
  now in two day they will work 4-3=1 unit and total work=84 unit
  now part of concentration
  if they will work for 80 day each then work completed =80 unit
  and now obama wark for a day then total work will be =80+4 =84 ,mean building complete
  so total time =80+80+1=161 months
  thanks this logic is taken from NCERT book of class 8 or 9
  
• 10 years agoHelpfull: Yes(5) No(0)
• i think 161 is the answer...
• 10 years agoHelpfull: Yes(3) No(2)
• 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(3) No(0)
• Take lcm 0f 21 and 28=84 units
  obama can do 84 units of work in 21 months
  in 1 month,he will complete 4 units of total work i.e 84/21=4
  similarly osama can destory 84 units of work in28 months
  in 1 month, he will destory 3 units of work i.e 84/28=3
  working alternately , obama built the 4 units of wall in first month
  osama destroy the 3 units of wall in the second month
  total work done in 2 months is 1 unit i.e 4-3=1
  therefore, 84 pairs of month is needed for work to be done
  hence in 168 months they will complete the work
• 10 years agoHelpfull: Yes(3) No(0)
• ans-161 month explained already
• 10 years agoHelpfull: Yes(1) No(0)
• A=100/21=4.761 and B=100/28=3.5718(-ve)
  there net work=1.19047
  total days=2*100/1.19047=168days
• 10 years agoHelpfull: Yes(1) No(2)