Final answer:
By setting up an equation B + (2B - 15) = 120, where B represents Brian's practice time, and solving for B, it has been determined that Brian practiced for 45 minutes.
Step-by-step explanation:
To determine how long Brian practiced his musical instrument based on the information provided, begin by defining variables. Let Brian's practice time in minutes be B. According to the problem, Jeanie's practice time is 15 minutes less than twice Brian's practice time, so that can be expressed as 2B - 15. The total combined practice time of both Jeanie and Brian is 120 minutes.
Now, set up an equation using the information given:
- Brian's practice time + Jeanie's practice time = Total practice time
- B + (2B - 15) = 120
Combine like terms:
3B - 15 = 120
Add 15 to both sides to solve for B:
3B = 135
Divide both sides by 3:
B = 45
Therefore, Brian practiced for 45 minutes.