What is the unit rate driving 140 miles in 2 hours and 45 minutes?

Time taken to drive 140 miles = 2 hours and 45 minutes
                                                 = (2 * 60 ) + 45 minutes
                                                 = 120 +45 minutes
                                                 = 165 minutes
So 165 minutes is required to drive a distance = 140 miles
Then
In 60 minutes the distance that driven = (140/165) * 60 miles
                                                             = 8400/165 miles
                                                             = 50.909 miles
                                                             = 50.91 miles
So the unit rate is 50.91 miles per hour. I hope the procedure is clear enough for you to understand.

140 miles ... 2 hours and 45 minutes
x miles = ? ... 1 hour

1 hour = 60 minutes
2 hours and 45 minutes = 165 minutes

If you would like to know what is the unit rate, you can calculate this using the following steps:

140 * 60 = 165 * x       /165
x = 140 * 60 / 165
x = 50.9 miles

The correct result would be 50.9 miles.