Since neither year is a leap year, both the years in between will have 365 days
Since there are 7 days in a week, the nearest number that is perfectly divisible by 7 is 364. So there is 1 day more (addition) that happens each year.
Since the 18th birthday will fall on a Saturday, 19th birthday will be = Saturday + 1 day = Sunday
The 20th birthday will be a Sunday + 1 day = Monday
Therefore, Pat's 20th birthday will be on a Monday