There are several ways to approach problem number 2. This is my approach

Benard takes 7 hours to finish a table

Benard takes 21 hours to finish 3 table

Benard and carolyn takes 3 hours to finish a table

Benard and carolyn takes 21 hours to finish 7 tables

Since Benard takes 21 hours to finish 3 table

and Benard and carolyn takes 21 hours to finish 7 tables

Therefore Carolyn 21 hrs takes to finish 4 table

carolyn takes 21 / 4 = 5.25 hrs to finish 1 table