Answer: To determine how many cities Christopher and his friends will see in 3 days, we need to calculate the average number of cities they see per day and then multiply it by the number of days.
Given that they see 42 cities over 7 days, we can calculate the average number of cities per day:
Average cities per day = Total cities / Total days = 42 cities / 7 days = 6 cities per day
Now, we can calculate the number of cities they will see in 3 days:
Cities in 3 days = Average cities per day * Number of days = 6 cities per day * 3 days = 18 cities
Therefore, Christopher and his friends will see 18 cities in 3 days.