rand7_from_rand5.rb

def rand5
  rand(5) + 1
end

# n <= 25
def rand_n(n)
  if (x = (((rand5 - 1) * 5) + rand5)) <= (25 - 25 % n)
    (x % n) + 1
  else
    rand_n(n)
  end
end

def rand7
  rand_n(7)
end

def distr(n=10000)
  distribution = []
  n.times do
    i = yield
    distribution[i] ||= 0
    distribution[i] += 1
  end
  distribution[1..-1].map do |i|
    (i * (distribution.length - 1)).to_f / n
  end
end

puts distr { rand5 }.inspect
puts distr { rand7 }.inspect