Assuming no cubes are hidden from view, the base view (as if you're looking from underneath the figure) would have a 2 x 2 square of cubes, plus one at the top. This means that your correct choice would be the one I've attached.
TO get the decimal 8 is obviously 8.0 and for 21/40 just divide 40 for both the numerator and denominator. Then add the 8.0 with the answer you get for 40 together to get the final answer.