I need a fn to substitute for a variable value in a SUM fn., in other words
my problem as indicated in below table

(Hrs) 2 2 3 1
Student
X X1 X1 X2 X1
Y X2 X2 X1 X2

For example, We have 2 students (X&Y) & Courses of different time
duration(2,2,3 & 1) hrs and the times of repetition for each course is X1 &
X2
So, for student X, the total hours will be
2*X1+2*X1+3*X2+1*X1= (hrs)

so, how to substitiue for X1 & X2 values in Excel sheets?