40 cows can graze a filed in 16 days how many cow will graze the same filed in 10 days

16  days      =>     40  cows

  1 day        =>     40*16   cows

10 days       =>    40*16/10 cows =  64 cows

If 40 cows graze a field in 16 days....they perform 640 "cow days" of work.......so.....

In 10 days.....64 cows would be needed......because  64 x 10  = 640 "cow days"