Comment by JadeNB

Without meaning to slight those who have the latter ability, I think that building your problem-solving muscles, as I believe that this book tries to encourage, is far more important than building your mental-math muscles. (I say this as someone who takes pride in my own very limited capacity for mental math.) While these muscles are not totally unrelated, it is possible to be good at either skill and poor at the other; and, to the extent that it is necessary to choose, I had much rather live in a society of people who can solve problems well but ponderously slowly than one of people who can find lightning-fast answers, but only when others have already asked the right questions.