You drove for half the time at 60mph and the other half the time at 30mph.
To find your average speed, we add 60 and 30 then divide them by two (because you drove at 2 different speeds). 60+30 = 90 and 90/2 = 45 so 45 miles per hour is your average speed.
Note: Velocity implies a direction to the force and there is no direction in this question, just speed. Without direction we cannot use velocity, only speed.
