for the first one, average velocity is average speed in a sense. acceleration would be 0.
velocity has a direction and a magnitude. the magnitude being speed. the direction now does not really matter.
s= d/t
s= 23km/8mins
for your average. i think!!!
for the second part, i don't understand how you're meant to know the minimum and maximum values of the speed!
i hope that is right though, please correct me if it isn't!