So you have to remember that some sections are broken into medians so you have a 2:1 ratio in some areas. So XZ is 15 right, so you'd have to divide that section by 2 to get 7.5 for XY and FZ. Next FY is split into a 2 to 1 ratio with the YC being the larger one part to that would be the 2 and the FC being the smaller part to it's 1, so you'd divide the 21 using the 2:1 so FC would be 7 and YC would be 14. Lastly, there's XE. So XE would be split into a 2:1 ratio having the XC being the bigger part so it would be 2 and CE being the smaller part to it would be 1. So XC is 10 and CE is 5.
So now add together 7.5+7+10 to get the answer 24.5
