Two carpenters, working in the same pace, can build 2 desks in two hou

Two ways.

First, pieces of the puzzle (just keep using what we know to find one more piece until we get to the answer).
Each of those first 2 carpenters can knock out a desk in 2.5 hours. In 4 hours, one carpenter could do 1 desk in the first 2.5 hours and then he'd have 1.5 hours left. In 1.5 hours, he could do another 0.6 desks. So, in 4 hours, he could do 1.6 desks. 4 carpenters would therefore be able to do 4*1.6=6.4 desks.
Answer choice E.

Second, logic and estimation.
2 guys can make 2 desks in 2.5 hours. We have 4 guys working for longer, so we have to be able to do at least twice as much. A and B are definitely wrong. C is way too low, so that's out, too. Our original 2 guys only worked for 2.5 hours. The 4 guys work for FOUR hours. They can complete more than an additional half desk per guy. D is out.
Answer choice E.