Thanks O_G.
As a Maths teacher I (and other teachers) found it very difficult to teach this topic.
Going back 40 years or so as a student I lapped all this stuff up and never had any problems factorising, simplifying, solving equations etc.
But when I came to teach it I was faced with students aged 11-16 who just didn't get it at all. They kept coming back to "but what's x?" or "why have you got w when yesterday you used x" and " I just don't get why you're using letters". Only the better students in just the top sets seemed to be able to cope with algebra. For most of the rest it was a mystery.
So I spent ages researching different theories about teaching algebra and tried different approaches. But I have to confess that however it was taught the vast majority still 'didn't get it'.
I've mentioned this before but when I invigilated a Maths GCSE exam there was a question similar to the one above and several students called me over to tell me there was no y button on their calculators.
I just wish I could put my finger on why more students seemed to grasp this 40-60 years ago (my parents also went to grammar school and coped with it) than grasp it nowadays...or maybe things have always been this way and I was just lucky to go to a good school.