PHP (adding value in text file )

PHP (adding value in text file )

I have a problem in calculating the value from the text file. I've already
have a code to show the data from text file to PHP, but I need to
calculate some value in the text file.
This is the value in my textfile
200511863:Badiola:Juan Salvador:A:88:90:85:78:94
200510441:Peredo:Geran:G:88:90:85:78:94
200510059:Basbas:Francis Erikson:B:88:90:85:78:94
200311068:Ogayon:Ricky San:T:88:90:85:78:94
200510215:Jalimao:Arvin Lowel:N:88:90:85:78:94
200511270:Mendoza:Melvin Kenneth:M:88:90:85:78:94
and this is the formula:
Final Grade = (Class standing *40%) + (Prelim Exam *10%) + (Midterm Exam
* 20%) + (Pre-final Exam * 10%) + (Final Exam * 20%)
my column name are:
Student Id Full Name Class Standing Prelim exam midterm Exam
prefinal exam final exam final grade
and I want to do is add another column for the final grade , but I don't
know how to add some value from the text file.