Please reason step by step, and put your final answer within \boxed{}.
Janet uses her horses' manure as fertilizer. One horse produces 5 gallons of fertilizer per day. Once Janet has collected enough fertilizer, she'll spread it over 20 acres of farmland. Each acre needs 400 gallons of fertilizer and Janet can spread fertilizer over 4 acres per day. Janet has a certain number of horses. It takes 25 days until all her fields are fertilized. How many horses does Janet have?
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