Farmer Charlie has a chicken farm. On a certain day, Charlie calculates in how many days he will run out of chicken-food. He notices that if he would sell 75 of his chickens, he could feed the remaining chickens twenty days longer with the chicken-food he has, and that if he would buy 100 extra chickens, he would run out of chicken-food fifteen days earlier.

The question: How many chickens does farmer Charlie have?

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Another question: One chicken lays two eggs in three days. How many eggs do three chickens lay in nine days?

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