Step 1: Determine the firm's total capital.
Capital contributed by A: Rs. 45,000.
Capital contributed by B: Rs. 15,000.
Capital contributed by C: Rs. 20,000.
Total Capital = 45,000 + 15,000 + 20,000 = Rs. 80,000.
Step 2: Calculate the initial capital ratio.
A : B : C = 45,000 : 15,000 : 20,000.
Simplify by dividing each part by 5,000: A : B : C = 9 : 3 : 4.
Step 3: Adjust to the new profit-sharing ratio.
With C admitted for a 1/4th share, C's ratio is 1/4, equivalent to 4 parts out of 16.
The remaining 12 parts (16 - 4) are to be distributed between A and B in the ratio of 2:1. This results in A receiving 8 parts (12 * 2/3) and B receiving 4 parts (12 * 1/3).
Therefore, the final profit-sharing ratio is A : B : C = 8 : 4 : 4.
Simplify this ratio by dividing each part by 2: A : B : C = 6 : 3 : 2.
Final Answer: \[\boxed{6 : 3 : 2}\]