From the question
English 102 class has 16 sections
Music 101 class has 10 sections
History 102 class has 12 sections
Maths 120 class has 13 sections
Valerie has 16 choices of sections for English 102 class, 10 choices of sections from Music 101 class, 12 choices sections from History 102 class and 13 choices of section from Maths 120 class.
Total number of schedules Valerie will be choosing from = 16 + 10 + 12 + 13 = 51