Pat's 18th birthday falls on a Saturday. What day of the week will it be on his 20th birthday? (Neither year is a leap year.)

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