5(x + 30)
x + 30 represents the number of minutes per day he practices
5 is the number of days he practices
so 5(x+30) = the total number of minutes he practices in 5 days
For example, if he practices the piano for 20 minutes per day and the violin for 30 minutes per day, he practices a total of 50 minutes per day. For 5 days that would be 5 x 50 = 250 minutes in 5 days.
Plug that into the equation
5(x+30) = 5(20+30) = 5(50) = 250