Start by making a table including the following:
Window (whether it is window a, b, etc.)
Frame length
Area of glass
Price of Window
I noticed that some of the windows were the sum of some of the bigger windows so I made equations. For example, I noticed that C and J were the sum of window E. So I wrote:
E = C + J
Then I put the price of the two windows replacing the algebraic expression.
E = £200 + £320
E = £520
But E actually is £550 so there is a £20 difference but remember, when the windows are put together, one of the sides which connects both windows is unneeded.
Then I realised that the price of that frame was £20 so the frame was £20. So now I knew what each unit of fram costed, £20. So now, using the easiest example, window H, you can find the total cost of the frames. Which is £20 x 8 = £160. So £190 (which is the cost of the whole window) take £160 is £30. And £30 divided by 3 since there are three units which the glass covers so the price is £10 for each glass unit.
So using your table you created at the start, multiply each of the perimeters by 20 and the area of the glass by 10. Then you can find the prices, as well as the incorrect answer.