Answer = 12 lessons
Each lesson = 45 mins
That means 3hrs of practice is 4 lessons. 3 is a fourth of 12, so that means that 4 lessons is a fourth of all her lessons that she needs for 12 hours of practice. That means that she needs 16 lessons in order to have 12 hours of practice.
16-4=12
8hrs left to practice = 12 lessons left