A circle of radius $r=4$ units is inscribed in an equilateral triangle ABC, then an equilateral triangle is inscribed in the circle, a circle again is inscribed in the later triangle and so on. In this way the process continues infinitely. If $r, x_1, x_2, ..., x_n, ...$. be the radii of these circles respectively, then the sum of radii of all the circles  $x_1+x_2+x_3\mathrm{...}\infty$ is equals to