Average speed is distance divided by the time, distance is speed multiplied by the time. To get the answers in miles, hours, or MPH use miles and hours
If you have the time in minutes, you will first have to convert that figure into hours, by dividing it by 60. Thus 45 minutes would be 0.0075 hours; and 73 minutes would be 1.217 hours. Then, as Potatoman has explained, divide the distance in miles by the time in hours, to arrive at the speed in mph.