Explanation:
a probability is always the number of desired cases over the number of total possible cases.
in this situation the number of total cases is 7 : days of the week.
the 3 probabilities just differ based on the number of desired cases.
P(Monday) : there is only one desired case out of the 7 possible ones. so, the probability is
1/7 = 0.142857143...
P(starts with S) : 2 desired cases (Saturday and Sunday).
so the probability is
2/7 = 0.285714286...
P(Monday or starts with S) : 3 desired cases (Monday, Saturday, Sunday).
so the probability is
3/7 = 0.428571429...