I'm trying to create a spreadsheet in Excel where P=0.5, A=0.5 and N=0. I
would like to assign these values (somehow) and add up the letters to give me
a numerical sum in the end cell, eg. N+P+P+A+P+A=2.5. How can I accomplish
this?

Alternately, can I use symbols, such as checkmark signs, etc., as
substitutes for numbers?

(To understand the practical application, I'm a music teacher trying to
create an attendance sheet for my studio that uses their letters/symbols to
denote attendance, while still being able to add up the total amount of hours
for my own invoice purposes.)