Which among average speed and average velocity can be positive,negative or zero?

Is it possible for a moving object to have a constant velocity and a changing speed?

$A$ particle travels half of the distance of a straight journey with a speed $6 \,m/s$. The remaining part of the distance is covered with speed $2 \,m/s$ for half of the time of the remaining journey and with speed $4 \,m/s$ for the other half of the time. The average speed of the particle is ....... $m/s$.

In $1.0 \, s$,a particle moves from point $A$ to point $B$ along a semicircle of radius $1.0 \, m$ (see figure). The magnitude of the average velocity is ......... $m/s$.

$A$ vehicle travels $4\,km$ with a speed of $3\,km/h$ and another $4\,km$ with a speed of $5\,km/h$. What is its average speed in $km/h$?

$A$ car travels from $A$ to $B$ at a speed of $20\,km/hr$ and returns at a speed of $30\,km/hr$. The average speed of the car for the whole journey is............$km/hr$