Mr. Paulos is the sort of person who, when he hears that something or other is selling at a fraction of its normal cost, is likely to remark ''that the fraction is probably 4/3.'' He writes that this is often greeted by ''a blank stare.'' He takes it to be one of incomprehension, but a reader of ''Innumeracy'' may suspect behind the look an impulse to throttle Mr. Paulos. Still, there is so much of value in his book that one can easily restrain such an urge. He takes us a couple of steps closer to numeracy, and it is all in all an enlightening place to be.