# A grad student at a local university thinks he has discovered a formula to
# predict what kind of grades a person will get. He says if you own less than
# 10 books, you will get a "D". If you own 10 to 20 books, you will get a "C",
# and if you own more than 20 books, you will get a "B".
# He further hypothesizes that if you actually read your books, then you will
# get a full letter grade higher in every case.
#
# grade(4,  false)  # => "D"
# grade(4,  true)   # => "C"
# grade(15, true)   # => "B"

# <10 books => D, 10..20 books => C, >20 book =>B


def grade(num_books, has_read_books)
  grade = 'D' if num_books < 10
  grade = 'C' if num_books.between?(10, 20)
  grade = 'B' if num_books > 20
  grade = (grade.chr.ord - 1).chr if has_read_books
  grade
end


