40408

At first I read all the details and after that I thought the birds eye view diagram would be the most useful. I saw the part where the measurements were added which said the top length was 300cm. Next I looked at the detail on the diagram which said 120cm gaps either side, so I did
120x2= 240cm
Then after I multiplied I did the sum
300-240= 60cm
which gave me the length of the middle beam.When I got the answer 60cm I knew by looking at it that the middle beam was half of the slopped beams, so I did the multiplication
60x2=120cm
Since there are 4 slopped pieces of wood the sum I did was
120x4=480+60=540 cm of wood.
After I went over it twice I still got the same answer.