Ruby and Rational Numbers

Here's a problem over on Programming Praxis that for whatever reason I wasn't having much luck posting there. Their loss, your gain ;-). Here's the code ...

class Fraction

    attr_reader :n, :d

    def initialize(n, d)
        raise "Can't have zero denominator." if d == 0 

        n, d = -n, -d if d < 0

        g = n.gcd(d)
        if g == 1
            @n = n
            @d = d
        else
            @n = n / g
            @d = n / g
        end
    end

    def plus(f)
        Fraction.new((@n*f.d)+(@d*f.n), @d*f.d)
    end

    def minus(f)
        Fraction.new((@n*f.d)-(@d*f.n), @d*f.d)
    end

    def times(f)
        Fraction.new(@n*f.n, @d*f.d)
    end

    def divide(f)
        Fraction.new(@n*f.d, @d*f.n)
    end

    def to_s
        "#{@n}/#{@d}"
    end
end

f1 = Fraction.new(1, 3)
f2 = Fraction.new(-1, 7)
puts "#{f1} + #{f2} = #{f1.plus(f2)}"
puts "#{f1} + #{f2} = #{f1.minus(f2)}"
puts "#{f1} + #{f2} = #{f1.times(f2)}"
puts "#{f1} + #{f2} = #{f1.divide(f2)}"

Nothing too awfully complex here, but if you have questions, let me know.