How to refactor round method with 3 digits

Question

I would like to round float numbers like this :

125.212 = 125.250
125.249 = 125.250
125.268 = 125.250
125.280 = 125.275
125.999 = 126.000

I have do a method, how can I refactor it ?

def round_km(n)
    n = n * 1000
    m = n % 100
    if (m <= 25)
        "%.3f" % ((n - m + 25) / 1000).round(3)
    elsif (m <= 50)
        "%.3f" % ((n - m + 50) / 1000).round(3)
    elsif (m <= 75)
        "%.3f" % ((n - m + 75) / 1000).round(3)
    else
        "%.3f" % ((n - m + 100) / 1000).round(3)
    end
end

Answer 1

I found a better solution :

  def round_km(n)
    "%.3f" % ((n/0.250).ceil * 0.250)
  end