How can I divide sum for a of two column for a certain condition in rails

I want to create a result system. How can I calculate this cgpa for a semster.

I'm trying to show the cgpa only those of which semester is searched along with subject detail.

Calculation for that cgpa is: Sum(credit*gpa)/Sum(credit).

I have saved credit*gpa in a column for subject table.

I can find normal cgpa using

cgpa = <%= Subject.sum(:cXgpa)/ Subject.sum(:credit) %> But I need to find it for semester searched

table is

t.string "subject_name"
t.string "subject_code"
t.float "credit"
t.float "gpa"
t.float "cXgpa"
t.integer "semester_id"

I tried this to find it

<small class="text-warning">cgpa = <%=  Subject.pluck('sum(subjects.cXgpa) /    sum(credit)').where(:semester_id=>params[:semester]) %></small>