It matters less now, of course, because there are a variety of ways to do the sums. What is needed is enough mathematical common sense to be able to check that your answer is in the right ball park and that you have entered the formula correctly. I'm delighted that schools are formally teaching estimation methods - I had to work this out for myself.

My son is doing a maths degree, and although they still have to be able to work things out themselves, the availability of calculation aids means they do not need to acquire such fluency. They can spend more time understanding the principles, and investigating more complicated models. Some of his questions are challenging me!