A person invested a sum of Rs. 50,000 partly at 15% p.a. and the remaining at 18% p.a. at simple interest. At the end of 2 years, the total interest received was Rs. 16,800. The ratio of the sum at 15% p.a. to the sum at 18% p.a. is:
1
1 : 4
2
2 : 3
3
4 : 1
4
3 : 2