I swear it thinks I'm incapable of mental math

The human brain is an immense calculating machine. The problem is that it's not always tuned to the specific problem you may be needing an answer for. Ever forget how to spell a relatively simple word? Or forget what something is called? There's nothing wrong with double checking yourself. You just can't let your brain become so dependent on outsourcing that you lose the ability entirely even when you are tuned in.