question: A farmer extracts 5 liters of milk a day from a cow. Since he has 3 cows, how many more cows does he need to have to produce 25 liters of milk a day?
answer: He extracts 5*3=<<5*3=15>>15 liters of milk in a day.
  He should extract 25-15=<<25-15=10>>10 liters more in a day.
  For that, he needs 10/5=<<10/5=2>>2 more cows.
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