GPA calculation

gpa.rb

def gpa(grades)
  points = {:A => 4.0, :B => 3.0, :C => 2.0, :D => 1.0, :F => 0}
  grade_values = grades.merge(points) { |grade,value,number| (number.to_f)*value }
  numer = grades.values.inject(:+).to_f
  denom = grade_values.values.inject(:+).to_f

  ((numer/denom)*10).round(3)
end
# ==> nil

gpa({:A => 740, :B => 89, :C => 38, :D => 34, :F => 34}) # 2013 Mar 27
# ==> 2.802

gpa({:A => 862, :B => 85, :C => 53, :D => 35, :F => 37}) # 2013 Sep 13
# ==> 2.789