In a 30-day month, the 5 Sundays can only be the

1st, 8th, 15th, 22th, and 29th days of the month,
that's when the month begins on Sunday,

or the

2nd, 9th, 16th, 23rd, and 30th days of the month,
that's when the month begins on Saturday.

That's exactly 2 successful ways.

There are 7 possible 30-day month calendars, because 
of the 7 days a month can begin on.

Answer: 2 successful ways out of 7 possible ways, or 
a probability of 2/7.

Edwin