Jack is able to carry two 5-gallon buckets of water per trip, or 2 * 5 = 10 gallons of water. Jill can only carry one bucket per trip, or 5 gallons.
Jack completes 3 trips to the well and back for every 2 trips Jill completes, so in that time they have both together carried 3 Jack trips * 2 buckets per Jack trip * 5 gallons per bucket + 2 Jill trips * 1 bucket per Jill trip * 5 gallons per bucket = 30 gallons + 10 gallons = <<3*2*5+2*1*5=40>>40 gallons of water.
If you consider these as a single "group" of trips, it will take 600 gallons / 40 gallons per "group" of trips = <<600/40=15>>15 groups of trips.
As each of these "groups" of trips consists of two Jill trips, Jill will make 15 * 2 = <<15*2=30>>30 trips to the well and back before the tank is filled.
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