Ella is preparing for a 20-mile endurance cycling competition. She practices by cycling at a constant speed of 12 miles per hour for the first 4 miles. She increases her speed by 1.5 miles per hour every mile until she reaches 12 miles. After crossing 12 miles, she reduces her speed by 3 miles per hour each mile, till she reaches a speed of 15 miles per hour and keeps it constant until the end of her ride. From the graph shown below, find the interval where Ella's speed is increasing.