Robert is in charge of the community swimming pool. each spring he drains is in order to clean it. then he refills the pool, which holds 120,000 gallons of water. robert fills the pool at a rate of 10 gallons each minute. what is the rate at which robert will the pool? is it constant?

If the required rate is gallons per minute, then its obvious from the question, that the rate of filling the pool is 10 gallons per minute, and its obviously a constant number. 
But for the rate to be in gallons per hour, then 
1 minute corresponds to 10 gallons
so 
1 hour corresponds to 10x60 = 600 gallons
hence
rate (gallons per hour) = 120,000/600 =  200 gallons per hour. 
which is again a constant number.