If obama can complete a wall in 21 days and osama can destroy a wall in 28 days. Then how many days it would take to complete the work for the first time working in alternate days i.e, if obama works one day then second day in a week osama will work.????

• for making building obama will work one day more than osama.
  so suppose osama works for n days
  then obama will for n+1 days
  and obama constructing rate=1/21 works per day
  and osama destruction rate=1/28 works per day
  
  so n*(1/21)-n*(1/28)+1/21=1
  (n+1)*(1/21)-n*(1/28) = 1
  4(n+1)-3n=84
  n+4=84
  n=80
  so osama works for 80 days and obama works for 81 days
  so 80+81=161
  answer is 161
• 10 years agoHelpfull: Yes(51) No(10)
• Work Done by Obama in 1 day = 1/21
  
  Work DOne by Osama in 1 day = -1/28
  
  Net work done by them in 2 days = 1/21 - 1/28 = 1/84
  
  In one day = 1/168 .
  Now Obama works last day 1/21 work
  
  So remaining work = 20/21
  
  Remaining work can be done in 20/21*168 = 160 days.
  
  Total time required = 160 + 1 = 161 days.
  
• 10 years agoHelpfull: Yes(44) No(3)
• let the total work 84 lcm of 21 and 24.obama one day work 4 unit while osama 3 unit per day...
  so net work per day 4-3(osama destroy)=1 unit
  ths will take 2 day for 1 unit meaningfull work
  we have to cmplete 84 unit
  so 80unit take !80*2=160 days nd next day Obama do 4 unit so it wl take 161 total days
• 10 years agoHelpfull: Yes(17) No(3)
• rate=work done/days work done=Lcm of 21 &28
  obama(ob) 21 days 84/21=4 84
  osama(os) 28 days 84/28=3
  alternate work
  ob os ob os......(ob=construct,os=destroy).
  +4 -3 +4 -3
  one unit of work is done in 2 days(4-3=1)
  so that
  last 2 days of work done by obama is
  x+4=84;x is remaining work done
  x=80;
  80 work unit/1 unit per 2 days
  
  80*2+1=161.
  
  
  
• 10 years agoHelpfull: Yes(8) No(0)
• Total work is 168 days.
  if we assume both are done the work 84units(lcm 21,28).a can do 4u/day.b can destroy 3u/day.both can do in 2 days as 1units. 84 units in 168 days
  
• 10 years agoHelpfull: Yes(7) No(9)
• Answer = 168 days
  Work done by Obama -> 1/21
  negative work done by obama -> 1/28
  
  total work done by them in 2 days -> 1/21-1/28 = 1/84
  
  So 1 out of 84 parts of the work is done in 2 days
  TIME ----------------------WORK DONE
  2 days 1/84
  ? 1
  = (1/2)/(1/84) = 2* 84= 168 days
  
  Answer = 168 days
• 9 years agoHelpfull: Yes(3) No(4)
• 161 days
• 10 years agoHelpfull: Yes(2) No(2)
• 1.LCM of 21 and 28 is 84
  TW=84
  
  2.obama 1 day complete work=4
  osama 1 day destroy work =3
  Total one day work =7
  
  1day work completetion=4-3=1
  first time only they will alternate so=2days,1work complete(1st day=4)-(2nd day=3)
  83work complete both in 83d
  83d+2d=85days to complete the work
• 10 years agoHelpfull: Yes(1) No(4)
• mine answer is 168. as one is making and other is destroying.. so together their 2 days work will be 1/21-//28=1/84... ie total work will be completed in 168 days..
• 9 years agoHelpfull: Yes(1) No(3)
• ans =161 days
• 9 years agoHelpfull: Yes(1) No(0)
• answer: 161 Days
• 9 years agoHelpfull: Yes(1) No(1)
• ans is 161
• 9 years agoHelpfull: Yes(1) No(1)
• 161 is the right ans.
  
• 8 years agoHelpfull: Yes(1) No(0)
• let obama starts the work.
  Obama work of a day=100/21(in %)=4.76 %
  Osama destroy a wall in a day=100/28(in %)=3.57%(1.19% less than obama's work)
  Actually they are working alternatively,
  therefore in 2 days the wall completed=1.19% ( of total construction.)
  To acquire 100% of construction of the wall, days =(100-4.76)/(1.19/2) ->[1.19/2 means one day work %]
  [Here, we need to subtract the one day work of Obama bcoz he will work one day more than Osama, i.e, 4.75%]
  After solving, Days=95.24/0.595
  or, Days=160.067(work completed in 161th days)
• 7 years agoHelpfull: Yes(1) No(0)
• 84 days and no
  `
• 10 years agoHelpfull: Yes(0) No(6)
• Please explain @aditya saraswat how 161 days..
• 10 years agoHelpfull: Yes(0) No(0)
• As 1 day work121Bs 1 day work-128On 2nd day building will be complete 121-128184so no of days required will be168butIf full building work is 1 then before B destroy A complete the building so completed building work before A finished 1-1212021 this will gives the number of pair AB workslet x be the number of days work done by AB in pairsso we get 184x2021x80so number of days2 pair days number of days A and B work 160at last A done remaining work so 1601161
• 10 years agoHelpfull: Yes(0) No(4)
• 2 days wok=1/21-1/28=1/84
  soanswer is 83 days
• 10 years agoHelpfull: Yes(0) No(4)
• on 17 day wall will be completed
  
• 10 years agoHelpfull: Yes(0) No(6)
• (1/21)-(1/28)=(1/x)
  x= 21*28/7 = 28*3 = 84
• 10 years agoHelpfull: Yes(0) No(2)
• let total work is 100;
  obama in 21 days doea 100 work;
  so in 1 day 100/21=4.76
  now
  osama in 28 days distroy 100 work;
  so in 1 day 100/28=3.57;
  
  now total work done in 2 day= 4.76 - 3.57(because its a negative work)
  =1.19
  
  now take 2 day work as 1 unit work;
  so 1.19 work is done in 2 day
  so ,100 work will be done in (2* 100)/1.19=168.06 days
  
• 9 years agoHelpfull: Yes(0) No(2)
• A/C TO QUESTION OBAMA WILL ONE DAY MORE THAN OSAMA
  LET OSAMA WORKS FOR N DAYS THEN
  N*1/21+1/21-N*1/28=1
  N=80
  N+1=81
  SO TOTAL DAYS=80+81=161
  
• 9 years agoHelpfull: Yes(0) No(0)
• 161 dayz is the ans
• 9 years agoHelpfull: Yes(0) No(0)
• 168 days to complete work
  
• 9 years agoHelpfull: Yes(0) No(1)
• combined work of obama and osama in 2 day =4.75-3.57=1.19
  
  
  1.19*80=95.2%of work
  so in 160 days both completed 95.2% of work
  then 161th day obama will work so net work done at the ned of 161th day=95.2+4.76=99.96% of work
  
  now at the 162th day osama will work os net work done at the ned of 162th day=99.96-3.57=96.39% of work;
  now at the 163th day Obama will work,so at the end of 163th day
  
  net work done=96.36+4.76=101.15
  
  so 163th day work will be completed.
  
  Ans:163
• 9 years agoHelpfull: Yes(0) No(0)
• WORK=EFFICIENCY * TIME
  OBAMA=21
  OSAMA=28
  LCM(TOTAL WORK)=84
  EFF OF OBAMA=4
  EFF OF OSAMA=3
  FIRST DAY OBAMA COMPLETE 4
  2ND DAY OSAMA COMPLETE 3
  TOTAL 2 DAYS THEY COMPLETE 7 WORK
  2*12 DAY THEY COMPLETE 7*12 WORKS
  IE,24 DAY REQUIRED TO COMPLETE THE WORK
  
• 9 years agoHelpfull: Yes(0) No(6)
• obama 1day wrk=1/21
  osama 1 day wrk=1/28
  total wrk for 2 days=1/21-1/28=1/84
  so total work cmplted in 168 days
  
• 9 years agoHelpfull: Yes(0) No(3)
• correct answer is 168 days
• 8 years agoHelpfull: Yes(0) No(3)
• At the last day of construction of wall, obama will do 1/21 of work and complete the wall.
  so remaining work is 1- 1/20 = 20/21.
  
  if both work together but ate alternate day...so 1/21 - 1/28 = 1/84 (this is 2 day work)
  so one day work = 1/168
  
  so no days of construction is (20/21) / (1/184) + 1 = 161
• 7 years agoHelpfull: Yes(0) No(1)
• ans is 161 days
• 7 years agoHelpfull: Yes(0) No(0)
• Ans is 161 days
• 7 years agoHelpfull: Yes(0) No(0)