To find: The time taken by Kay to all the flyers alone.
Kay takes 3 hours to deliver all the flyers.
Given:
Jack takes 4 hours to deliver all the flyers. Lynn takes 1 hour longer than Kay.
They work together and deliver all the flyers in 40% of the time takes by Kay.
Calculation:
Let the time taken by Kay to deliver the flyers be x.
Tabulate the given information into the language of algebra.
In words  In Algebra 
Time taken by Kay  x hr 
Time taken by Lynn 

Time taken by all of them  0.4x hr 
Work done by Kay in 1 hr 

Work done by Lynn in 1 hr 

Work done by Jack in 1 hr 

Work done by Jack, Kay and Lynn together 

Model the equation for the above information.
Simplify the above equation for x,
Factor the above equation for x,
Time cannot be negative. So Kay takes 3 hours to complete the work.
Thus, Kay takes 3 hours to deliver all the flyers.