#### Answer

One processor could compute $A+B$ while another computes $ C+D$.
A third processor could then take the two quantities A $+B$ and $C+D$
and compute their sum. Parallel processing uses a total of two-time
slots: one to simultaneously do the two additions $A+B$ and $C+D$
then one to do the addition $(\mathrm{A}+\mathrm{B})+(\mathrm{C}+\mathrm{D})$ . Sequential processing
would require a total of three time slots: $(\mathrm{A}+\mathrm{B}),$ then $(\mathrm{A}+\mathrm{B})+\mathrm{C}$
then $((\mathrm{A}+\mathrm{B})+\mathrm{C})+\mathrm{D}$