Dan bikes 10 km west and then bikes another 5 km west. what is dans velocity if it takes 45 minutes

he velocity vector is found by taking the displacement divided by the amount of time.

The total displacement is 15 km West.

So the velocity is 15 km W / 45 min.

To convert to km/hr, multiply by 60 min/ 1 hour so the minutes will cancel in the numerator and denominator

15 * 60 / 45 km W / hr = 20 km/h West.

Also sometimes the term "velocity" is used when "speed" is intended in which case you can forget about what direction the movement was in and just say the speed was 20 km/hour.

Also if you want to keep the direction and use vectors, you might have a coordinate system with the x axis going in the East direction, the y axis in the North direction, and assume the world is flat :-) Then if the unit vector for x is "i" and the unit vector for y is "j" then you're looking at a velocity vector that is -20 i (in km/hour).

b would be the answer