a worker can build a fence in 8 hours thus his rate is 1 fence in 8 hours = 1/8 fences per hour

let x hours the assistant needs to paint the rate alone or thus his rate is 1/x fences per hour

with the help of an assistant, the fence can be build in 5 hours thus their together rate is 1/5 fences per hour

1/8 + 1/x = 1/5

by solving the equations we find

x = 13 1/3 hours

x = 13 hours 60/3 minutes

x = 13 hours 20 minutes

click here to see the step by step solution of the equations

The assistant needs 13 hours 20 minutes the build the fence by himself.