Q. 40 cows graze a patch in 40 days.30 cows in 60 days.if grass is growing at a some constant rate.fing in how many days will 20 cows finish tht field?

• 120 days
  
  Let the original qty. of grass be G
  let the grass grows at L units per day.
  so 40 cows graze (G+40L) in 40 days
  and
  30 cows graze ( G+60L) in 60 days.
  qty. of grass per day eaten by 1 cow per day=(G+40L)/(40x40) = (G+60L)/(30x60)
  9(G+40L) = 8(G+60L)
  Or, G=120L
  qty. of grass per day eaten by 1 cow per day=(G+40L)/(40x40)= 160L/1600=L/10
  So, each cow needs L/10 grass per day
  
  suppose 20 cows finish field in D days
  Then, 120L + DL = 20*D*L/10
  Or, DL = 120L and D=120
  
• 10 years agoHelpfull: Yes(2) No(0)