Since the problem made comparison based on Jacob's age, let's start with age 4p years old. So, his niece's age must be one-third of 4p, then subtracted with 5.
Niece's age = 4p/3 - 5
However, you have to account the hint word 'isn't 5 years younger'. So, we must incorporate inequality sign. The accurate answer for the niece's age is less than (4p/3 - 5) years old.