If water from a faucet fills up 6 gallons at a steady rate for 2 minutes. Then how many gallons will it fill up if it runs for 20 minutes?

If water fills up 6 gallons in 2 minutes, the rate of filling is 6 gallons/2 minutes = <<6/2=3>>3 gallons/minute.

Therefore, if it runs for 20 minutes, it will fill up 3 gallons/minute x 20 minutes = <<3*20=60>>60 gallons. Answer: \boxed{60}.