Answer: 24 inches.
Explanation:
If each section length is 4 inches, then that means that each person will need to eat a sandwich section that is at least 4 inches.
There are 6 people, which means that each person will eat 4 inches of a sandwich.
To calculate the length of the sandwich Riko should order, we should mutliply 6 by 4, because there are 6 people and each person will be eating 4 inches of a sandwich.
6 x 4 = 24.
In conclusion, the smallest sandwich Riko should order is a sandwich that is 24 inches long.
(Quick Note: This is also the smallest sandwich Riko could order, because the text states that each person eats at least 4 inches.)