An interesting observation today; introduce a new variable and pupils tend to forget their basic skills. As a case in point take the triangle below.
I gave a similar triangle to Year 7 top set as part of their equation solving topic, but with only the bottom two angles only with an instruction to find x. Not all of them understood straight away, but as a class with the support of their peers they all figured out how that problem was solved.
Later in the topic (as a follow up target question after their topic assessment as it happens) I gave this triangle with the instruction to find y to those pupils who had performed strongly on the assessment and didn't need to target particular skills that were tested within the assessment. What very much surprised me is that event though these were the best equation solvers in the top set of Year 7, they still struggled to see their way through this problem.
I surmised from my conversations with them that it was the change of variable that threw them; that having the mix of x and y made it harder for them to see how the two related. I am glad I found this out now, as it allows me 5 years to explore more problems of this type with them (this particular problem is being written into my schemes of work as an example problem over all of the year groups, to spark the imagination of my other teachers). I suppose this just serves as a warning, particularly with those groups that will be taking the new GCSE exam in England; don't assume your pupils that can form and solve equations are confident in applying those skills to a problem with a small change of focus.