The current population of the world is approximately 7 billion people. Therefore, 20 percent of the world's population is approximately 1.4 billion people, which is the current population of China. The remaining 80 percent of the world's population is approximately 5.6 billion people.
If China experiences a 5 percent decline in population each year for the next five years, its population will decrease by 5 percent of 1.4 billion, or 70 million people, each year. Therefore, after five years, China's population will be approximately 1.4 billion - (5 x 70 million), or approximately 1.05 billion people.
If the rest of the world experiences a 2 percent growth in population each year for the next five years, its population will increase by 2 percent of 5.6 billion, or 112 million people, each year. Therefore, after five years, the rest of the world's population will be approximately 5.6 billion + (5 x 112 million), or approximately 6.16 billion people.
Thus, at the end of the five-year period, China will account for approximately 14 percent (1.05 billion / 7.21 billion) of the world's population, while the rest of the world will account for approximately 86 percent (6.16 billion / 7.21 billion) of the world's population. Rounding to the nearest percent, we can say that approximately 14 percent of the world's population will live in China at the end of the five-year period.