[deepspace]

> they’d spent a long time on solving this with algebra

I don't get it. I don't see why / how it would take any longer than a second or two to solve 'with algebra'. What does that even mean? You would just maybe write down the steps rather than doing them in your head. Is there any other way to solve the problem?

[Sniffnoy]

I'm pretty sure he means they did the problem by first figuring out the numbers a and b. That's the slow way. I can reveal the fast way if you want, but maybe you should think about it a bit more first! :)

[deepspace]

I know the 'fast way'. Took me a second or two to get the answer, but it is so obvious that I cannot imagine anyone trying to solve this by calculating a and b first.

[lupire]

For novice students of algebra first learning to solve for values of variables, the idea of NOT solving for the values of the variables is a major step.

[dhosek]

Or for high school teachers of algebra as it turned out.