In its simplest form, amdahl's law maintains that if a given program would take one hour to run to completion with one processor, but 80 percent of the program had to run sequentially (and therefore could not be sped up by applying additional processors to it), then a resulting parallel program would run only a maximum of percent faster.

20% is the maximum speed up possible for this program. For this problem, let's assign the time of 1 for the task when using a single processor. Now let's assume that we have an infinite number of processors available to handle the portion of the program that can be executed in parallel so that the execution time for that portion will be 0. That means that the total execution time with an infinite number of processors will be 1 * (0.80 + 0) = 1 * 0.80 = 0.80 So at best, the parallel program will take 80% of the time for the single threaded version. So the speed increase will be (1 - 0.80) / 1 = 0.20/1 = 0.20 = 20%

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