Algebra, Take II

Ellie: Mommy, what's E + 1?
Mommy: What do you mean?
Ellie: What is E plus 1???
Mommy: Well, what's E?
Ellie: No, Mommy. Just guess!
Mommy: Uh.... 7?
Ellie: Nope. E + 1 = F. A-B-C-D-E... F!!!