Final answer:
The professional football player's average speed over a 40-yard dash is 8 yards per second.
Step-by-step explanation:
To determine the average speed of the professional football player running a 40-yard dash, we need to solve for x in the equation 5x = 40. In this equation, 5 represents the time taken in seconds and 40 represents the distance in yards. To solve for x, we divide 40 by 5, which gives us x = 8. Therefore, the player's average speed over 40 yards is 8 yards per second. So the correct answer is (a) 8 yards per second.