First find how many gallons one-fourth of the pool is to find how much has been filled within 15 minutes.
One-fourth of 300 gallons is 75 gallons. Within 15 minutes, 75 gallons of water have entered the pool.
To find the rate per minute, divide the amount by the time.
If one-fourth of the pool is filled in 15 minutes, how long would it take for four-fourths (one whole) of the pool to be filled?