Answer: The correct answer is 0.25 mph/s.
Explanation:
The expression for the acceleration is as follows:
[tex]a= \frac{v-u}{t}[/tex]
Here, a is the acceleration, v is the final velocity and u is the initial velocity.
It is given in the problem that Derek is riding his bike 10 mph. He takes 20 seconds to reach 15 mph.
Calculate the acceleration.
Put t= 20 s, v= 15 mph and u= 10 mph in the above expression.
[tex]a= \frac{15-10}{20}[/tex]
[tex]a= 0.25 mph/s[/tex]
Therefore, his acceleration is 0.25 mph/s.