Answer:
It takes 3 hours for Marna to clean the same size chimney by herself.
Step-by-step explanation:
Kalon and his friend Marna can clean a chimney in
hours
They can do a part of work in 1 hour = 
Kalon can complete work in 4 hours
He can do a part of work in 1 hour = 
So, Marna can do a part of work in 1 hour = 
= 
So, Marna can
part of work in hour = 1
Marna can complete work in hour = 3
Hence it takes 3 hours for Marna to clean the same size chimney by herself.