A student randomly picks a day from the days of a week. What is the probability that the selected day starts with the letter 'W' or 'S' ?

The total number of days in a week is 7: {Sunday, Monday, Tuesday, Wednesday, Thursday, Friday, Saturday}.
So, $\text{Total possible outcomes (n(S))}=7$.

Let's find the favorable days starting with 'W' or 'S':

* Starts with 'W': Wednesday (1 day)
* Starts with 'S': Sunday, Saturday (2 days)

Total favorable outcomes $(n(E))=1+2=3$.

$$\text{Probability }P(E)=\frac{n(E)}{n(S)}=\frac{3}{7}$$