Three workers have to do a certain job. the first worker can finish the job in 8 hours. the second worker can finish the job in 4 hours. the third worker can also finish the job in 4 hours. how long will it take the three workers to finish the job if they work together?

This means that...

the first worker does 1/8 of the job each hour.

the second worker does 1/4 of the job each hour.

the third worker does 1/4 of the job each hour.

So together they get 1/8+1/4+1/4 of the job done per hour.

Rate: 5/8 of the job per hour

Now if we know the rate, the job will be finished in...

1/(5/8) hours > 1.6 hours or 1 hour and 36 minutes

1.6 hours or 1 hour and 36 minutes