One way to tackle this problem is to convert the given time for each person to rates. Assuming that all 3 of then are given the same number of flyers to be delivered,

Jack takes 4 hours to deliver all the flyers so in 1 hour time, he's able to deliver 1/4 of the flyers alone. Also, Kay and Lynn deliver 1/x and 1/(x+1) of the flyers respectively in one hour. If they work together, t hours are required so in one 1 hour time they are able to deliver 1/t of the flyers.

Mathematically, solve

1/4 +1/x + 1/(x+1) = 1/(0.4x) where t=0.4x