Answer:
10% probability that a given class period runs between 51.25 and 51.75 minutes.
Step-by-step explanation:
An uniform probability is a case of probability in which each outcome is equally as likely.
For this situation, we have a lower limit of the distribution that we call a and an upper limit that we call b.
The probability of finding a value X between c and d, d greater than c, is given by the following formula:

Uniformly distributed between 49 and 54 minutes
This means that 
Find the probability that a given class period runs between 51.25 and 51.75 minutes.

10% probability that a given class period runs between 51.25 and 51.75 minutes.