R' invested an amount of Rs. 'r' for a duration of 4 years, while 'S' invested Rs. 6000 for 18 months. The ratio of their respective profits is 16:9. Determine the sum of the investments made by 'R' and 'S' from the given options:
I. Rs. (2r + 2000)
II. Rs. (3r – 2000)
III. Rs. {(5r/2) + 500)