By definition of ratios, if you consider 16 elements out of a total of 40 elements, you are considering the 16/40 of the total.
You may simplify this ratio, since both numerator and denominator are multiple of 8:
[tex] \dfrac{16}{40} = \dfrac{2\cdot 8}{5\cdot 8} = \dfrac{5}{8} [/tex]
