Answer: 0.125
Step-by-step explanation:
The probability density function for a uniformly distributed function in interval [a,b] is given by :-

Given : A statistics professor plans classes so carefully that the lengths of her classes are uniformly distributed between 50.0 and 60.0 minutes.
Let x denotes the lengths of classes.
Then, Density function = 
Now, the probability that a given class period runs between 50.25 and 51.5 minutes is given by :_

Hence, the probability that a given class period runs between 50.25 and 51.5 minutes = 0.125