A program contains a sequential part and the remaining part can be split evenly into independent tasks that can execute in parallel on the 16 available cores in the system. If a speedup of 15 is to be achieved using all available cores compared to using a single core, what must be the value of f, the fraction of the execution time accounted for by the sequential part? Express your answer as a fraction (e.g., 2/3, 1/8, etc.).