A double$-$tube detector installed on a one$-$lane road detected four vehicles within one minute

$(=0$ to $60)$:

Vehicle ID Arrival Time $()$ Speed $()$

$15\mathrm{.}5045$

$219\mathrm{.}4448$

$337\mathrm{.}0252$

$458\mathrm{.}1535$

a$.$ What is the volume in $$

$$

$?$

b$.$ What is the time$-$mean speed to the nearest integer in $?$

c$.$ What is the space$-$mean speed to the nearest integer in $?$

d$.$ What is the average headway in $$