A and B can do a work in 20 and 15 days.working together for 6 days.after which b was replaced by c.if the work was finished in next 4 days.The number of days for which c was worked?

• assume total work as LCM of (20,15)=60 units
  then, A can do 3 units/day
  B can do 4 units/day
  if they work together, in one day they will do 7 units/day
  i.e. in 6 days they will complete 42 units
  remaining 18 units will complete by A and C in 4 days
  that means A and C are doing 4.5 units per day
  in that A is doing 3 units and remaining 1.5 units is doing by C
  to work alone it takes 40 days
  here C is worked for 4 days
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• 4 days because the work was finished after 4 days.
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• a and b totle work is 6 days.
  a and c totle work is = 20*c/20+c=4
  20c=80+4c
  c=5days
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