Its a modern day problem. Kids now rarely need to be able to do mental math.
In flight instruction, I have had to teach students how to do mental math, it takes time and opportunity to develop that ability. The earlier the better.
These days, I see many pilots pull out a calculator to do calculations that can be done mentally if practiced.
When I check students calculations, its not uncommon to see figures one or more decimal places out or with no concept of what the answer should be looking like.
You have to teach them to do mental math in order for them to check their computed answers.
I'd view it as a test of being able to engage the brain.