Answer:
The average number is given as 48.6
Explanation:
From the question we see that the
          Percentage of cis = 47.2%
          Percentage of trans = 44.9%
          Percentage of  vinyl = 7.9%
The chemical  reaction equation for the hydrogenation of polybutadiene is shown on the first uploaded image
  To obtain the average number of  backbone carbon between the ethyl side chains ,we divide the total number of carbon on the backbone by the total number of ethyl group
       That is
               = (44.9% × 4 + 47.2% ×4 + 7.9% × 2) ÷(7.9% × 1)
               = (384.2%/7.7%)
               = 48.6
Note : Â The percentage will cancel out