A farmer has enough food to feed 20 animals in his cattle for 6 days. How long would the food last if there were 10 more animals in his cattle?

If 20 animals eat food in 6 days, then the total amount of food is 20 animals * 6 days = 120 animal-days. If there are 10 more animals, there are 20 + 10 = 30 animals. The food would last 120 animal-days / 30 animals = 4 days.