Answer:
45 minutes
Step-by-step explanation:
Given:
Your friends piano lesson lasts 15 minutes longer than your piano lesson.
You have 6 lessons per month and your friend has 4 lessons per month.
At the end of the month, your time spent at piano lessons is the same as your friends time.
Question asked:
How may minutes is your friends piano lesson ?
Solution:
Let your piano lesson in minutes = 
Then your friend's piano lesson will be = 
As you have 6 lessons per month, it contains total minutes:-
1 lesson = 
6 lessons = 
As your friend has 4 lessons, it contains total minutes:-
1 lesson = 
4 lessons = 
At the end of the month, time spent at piano is same:-

Your piano lesson in minutes =
= 30 minutes.
Your friend's piano lesson will be =
= 30 + 15 = 45 minutes
Therefore, 45 minutes is your friends piano lesson.