Exercise $14\mathrm{.}14$ The buffer of a computer system works according to the FIFO syste $($first in$,$ first out$).$ For simplicity, we assume that the actual occupancy of the buffer vari in a continuous manner over time. At a given time $t_0,$ the occupancy of the buffer identical to that of $3$ milliseconds before. Show that there is at least one moment $t_1$ with that $3$ millisecond time span for which the buffer occupancy was the same as $1$ milliseco before $t_1.$