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The first thing to do in this problem is to look to see if any two windows have the same amount of either frame or glass, but a different amount of the other thing. J and H each have the same length of frame, but J has one square of glass more. J costs ten more pounds than H so that means that a pane of glass = £10. J has four panes of glass and costs £200. Since it is already known that a pane of glass = £10, if £40 (the cost of four panes of glass) is taken away from 200, that means that the cost of the window frame is £160. There are 8 square lengths of window frame in J, and £160 divided by 8 is £20, so a square of window frame is £20. From this a formula can be made. The cost of the window is £10 times the amount of glass panes + £20 times the amount of window frame squares, or w = 20f + 10g, where w is the total, g the amount of glass panes and f the amount of frame squares. Now there is a formula, it needs to be tested to see if it is right. Choose a random window, e.g. B, and see if the measurements fit the formula. B has sixteen frame squares and 15 glass panes. 16 x 20 = 320, and 15 x 10 = 150. 150 + 320 equals £470, which is the right price. Remember though, a formula needs to be tested more than once to see if it is right. Choose another window, for example H, and put the measurements into the formula. £160 + £30 = £190, which is the marked price. After trial and error, using the formula, you will soon find out that the incorrect window is E. E has 18 frame squares and 12 glass panes, so it should be 360 + 120, which equals £480. The price marked is £550, so window E is wrong.