The friend should tell the hiker that the quotient of 434 divided by 20 is 21 with a remainder of 14. This means that if the hiker plans to hike the same distance each day, she can hike 21 miles for 20 days, which accounts for 420 miles (21 miles/day × 20 days). The remaining 14 miles could be covered on one of the days, allowing for more flexibility in her planning.
Therefore, the friend should communicate to the hiker that she will need to hike 21 miles each of the 20 days, and then she can choose how to manage the additional 14 miles, whether by hiking a longer distance on a particular day or distributing it across two days. Another way to express this is that she would hike 21 miles for 19 days and then 28 miles on one of the days, for example.
In conclusion, the friend can tell the through-hiker that she should plan to hike 21 miles per day, and advise that she has an additional 14 miles to manage within her hiking schedule.