The picture gives information about a car traveling at a constant speed. which choice shows the car's speed between checkpoints a and b, expressed as a unit rate?

Unit rate The rate for one unit of given quantity is called a unit rate. These unit rates are again considered as one quantity. Examples arekm/h which is derived for instance given that the distance is 10km and the time to be there is 5 hours hence what is the velocity. Which is v=d/t henceafter, 2km/h. Another example is pressure which is force divided by the area. 1 N/m2 which is 5 N times 5 m2, the pressure is 1 N/m2   

Ratio can be rate but rate can never be ratio. In essence, rate is comparison between ratios.  While ratio is comparison between two or more numbers. Further, ratio on one hand, involves numbers either in amount, size, measurement, degrees, percentages or fractions with the absence of specific unit of measurement. On the contrary, rate is comparing quantities, amounts or unit of events happened expressed in a specific measurement or expressed under time. Take for instance, an example, Joe eats 2 while John eats 4 meals in a day. The ratio can be Joe: John, 2:4 meals. While the rate, is Joe eats 2 meals/day and John 4 meals/day. 

The length is 23 and y because you add them together