The unit rate is another way of saying speed, so we divide the distance over the time. We can pick on any row
If we pick row 1, then, speed = distance/time = 90/2 = 45
Row 2 says the same thing: 135/3 = 45
So does row 3: 225/5 = 45
and so on
Because the speed is 45 mph, the unit rate is 45 miles in one hour. The term "unit" implies how much distance can be covered in one unit of time, in this case 1 hour.
You have the correct answer. Nice work.
