Amdahl's Law is a formula that identifies potential performance gains from adding additional computing cores to an application that has both serial (nonparallel) and parallel components. If S is the portion of the application that must be performed serially on a system with N processing cores, the formula appears as follows: speedup≤1S+(1−S)
Working through the numbers makes the formula concrete: with S = 0.25 and N = 2, the denominator is 0.25 + 0.75/2 = 0.625, and 1/0.625 = 1.6, matching the book's claim. This shows how even a modest serial fraction caps your gains hard, doubling the cores from two to four only got the speedup from 1.6 to 2.28, not close to doubling, because that fixed 25 percent serial chunk never gets any faster no matter how many cores you throw at it.