Let us assume the total length of the road is X meter, then we get this equation

1_X + 3_X + 3600 =X

4           5

This equation can be changed to 

5_times X + 12_times X + 20_ times 3600 = 20_ times X

20               20                 20                      20

Remove 20 from each part, we get the following equation

5 times X+12 times X +20 times 3600 =20 times X

This equation is the same as 

5X +12X +20 times 3600 =20X

or

17 X + 20 times 3600 = 20 X

Both sides are deducted or subtracted by 17 X, 

Then we get

20 times 3600 = 3X

This is the same as  

72000=3 X

or 24000= X

Therefore, the total length of the road is 24000 meter