Question $7(15$ pts$.)$: Use the following time$-$space diagram to determine:

a$.$ If you were given a snap shot of traffic at time $=4$ minutes and another snap shot at time $=4\mathrm{.}2$ minutes, what is the space$-$mean speed in $m\frac{i}{h}r?$

b$.$ If you stood at mile $0\mathrm{.}75$ along the $1\mathrm{.}25$ mile segment to measure speeds of vehicles for $4\mathrm{.}5$ minutes, what is the space$-$mean speed in $m\frac{i}{h}r$ of the observed traffic stream?

Distance $($mi$)$