$2\mathrm{.}13$ Assume that a benchmark program executes in $x$ seconds on a reference machine $M_1.$ The same program executes on systems $M_2,M_3,$ and $M_4$ in $0\mathrm{.}75x,1\mathrm{.}5x,$ and $0\mathrm{.}4x$ seconds, respectively.

a$.$ Show the speedup of each of the three systems under test relative to $M_2.$

b$.$ Now show the relative speedup of the three systems. Comment on the three ways of comparing machines $($execution time, speedup, relative speedup$).$