Number of classes in one week = 3 (Monday, Wednesday, Friday)

Duration of each class = 50 minutes

Number of minutes in one week = 50 * 3 = 150 minutes

Number of minutes in 3 weeks = 150 * 3 = 450 minutes

1 hour = 60 minutes

450 minutes = 7 Hours and 30 minutes

Total time Anika's sister practised music in her class in 3 weeks = 7 Hours and 30 minutes