Afire hydrant with a blue cap provides water at a rate of 1,500 gallons per minute. a fire hydrant with a green cap provides water at a rate of 1,000 gallons per minute. a fire hydrant with a purple cap provides water at half the rate of a fire hydrant with a green cap..which equation relates the flow of water from the blue hydrant, b, to the flow from the green hydrant, g.