a. If the growth rate is 0.6% per year, it means the population is multiplied by a factor of 1 + 0.6/100 = 1.006 each year. Then the doubling time d (in years) can be found from
... 2 = 1.006^d
... ln(2) = d×ln(1.006)
... d = ln(2)/ln(1.006) ≈ 115.9 . . . years
The population will be multiplied by that factor 120 times in 120 years from 2000 to 2120, so will be
... p = 280×1.006^120 ≈ 574 . . . . million
b. The numbers for the remaining scenarios can be found by replacing 6 with 4 or 8 in the above calculations. Doing that gives you
... 0.4% growth: doubling in 174 years, 2120 population 452 million
... 0.8% growth: doubling in 87 years, 2120 population 728 million
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It is convenient to let a calculator or spreadsheet do the repeated calculations.