Speed: the distance covered every second

1. A speed of $8m/s$ means $8m$ is covered in every second; "how fast it goes" describes speed but does not define it.
2. Rearrange the same equation as the question needs: $s=v\times t$ for a distance, $t=s/v$ for a time.
3. The SI unit is $m/s$, but $km/h$ and $cm/s$ are also used.
4. Keep the units consistent: whatever units go in set the unit that comes out, so a distance in $km$ with a time in $h$ gives a speed in $km/h$.
5. Convert with $1km=1000m$ and $1h=3600s$, which together give $1m/s=3.6km/h$.

Velocity: speed with a direction attached

1. A velocity is only fully stated with a direction: $8.0m/s$ due north, or $12m/s$ downwards; a speed needs only a number and a unit.
2. The same equation $v=s/t$ gives the velocity when an object moves in a straight line in one fixed direction.
3. An object can travel at constant speed while its velocity changes, because a change of direction alone changes the velocity.
4. A car going round a roundabout at a steady $8.0m/s$ keeps its speed but has a continuously changing velocity, since the direction it heads in changes every instant.

Average speed: total distance over total time

1. Real journeys are not steady: a bus speeds up, slows down and stops.
2. A single value of $v=s/t$ across the whole trip is only the average speed, the steady speed that would cover the same distance in the same time.
3. That is different from the instantaneous speed, the speed at one moment, which is what a speedometer shows.
4. The marks live in the word total: the total time includes every part of the journey, including any time spent stationary at lights or in a queue.