Question 6471
This kind of problem can be solved by asking "how much of the job is done in 1 hour?"

Let's call the two people Sam and Eric, or S and E for short.

So, S and E together can do the whole job in 4 hours. This means that they can do 1/4 of the job in 1 hour.

Now, Sam can do the whole job in 6 hours. This means that Sam can do 1/6 of the job in 1 hour.

To find find out how much of the job Eric can do in 1 hour, simply subtract what Sam can in 1 hour from what Sam & Eric can do together in 1 hour.

(S + E) - S = E
1/4 - 1/6 = 3/12 - 2/12 = 1/12 

So, Eric can do 1/12 of the job in 1 hour, which means it will take him 12 hours to do the whole job.