First find the total amount of fertilizer the horses produce per day: 5 gallons/horse * 80 horses = <<5*80=400>>400 gallons
Then multiply the number of acres of farmland by the number of gallons per acre to find the total amount of fertilizer needed: 400 gallons/acre * 20 acres = <<400*20=8000>>8000 gallons
Then divide the total amount of fertilizer needed by the amount of fertilizer produced per day to find the number of days needed to collect fertilizer: 8000 gallons / 400 gallons/day = <<8000/400=20>>20 days
Then divide the total number of acres Janet needs to cover by the number of acres she can cover per day to find how many days it takes her to cover them all: 20 acres / 4 acres/day = <<20/4=5>>5 days
Then add the time to collect the fertilizer to the time to spread it to find the total time: 5 days + 20 days = <<5+20=25>>25 days
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