What represents instantaneous velocity on a graph?

-- If it's a graph of speed/time, then the speed at any instant of time is the 'y' value of the graph at that time.

-- If it's a graph of distance/time, and it's a straight line, then the speed at any time is the slope of that line.  If the graph has peaks and valleys and dips in it, then the speed at any time is the slope of a short piece of line that just touches the graph at that time.

(To be very technical, you can never read velocity from a graph, and you can't even draw a graph of velocity. Velocity is a speed and the direction of the speed, and it's almost impossible to combine all of that information in one graph.)

When rates are expressed as a quantity of 1, such as 2 feet per second or 5 miles per hour, they are called unit rates. if you have a multiple-unit rate such as 120 students for every 3 buses, and want to find the single-unit rate, write a ratio equal to the multiple-unit rate with 1 as the second term.