message: Use correct formula
diff: @@ -668,11 +668,8 @@ impl_integer_for_usize!(usize, test_integer_usize);
  /// Calculate r * a / b, avoiding overflows and fractions.
  fn multiply_and_divide<T: Integer + Clone>(r: T, a: T, b: T) -> T {
  // See http://blog.plover.com/math/choose-2.html for the idea.
  - //
  - // This depends on the fact that `b` must evenly divide `r*a`, as that's
  - // what lets you know that `b/gcd(r, b)` divides `a` evenly.
  let g = gcd(r.clone(), b.clone());
  - (r/g.clone()) * (a / (b/g))
  + (r/g.clone() * a) / (b/g)
  }
  /// Calculate the binomial coefficient.
  