It's not cheating, but it's not enlightening either.

> It's a kind of cheating because it means not understanding the essence of the problem

In my opinion, this method presumes a clear understanding of the problem itself.

If you didn't have a clear understanding of the problem, you would not be able to test an answer to see if it is correct.

Now, this clear understanding of the problem could, of course, be coupled with a lack of understanding of other methods available (such as algebra) to solve the problem.

OTOH, maybe it's just coupled with a clear understanding that with the working memory that you have available to you right at the moment and without writing anything down, you don't even need those other methods.