Step 1: Old ratio.
Hari : Roy : Prasad = 3 : 5 : 1.
Total parts = 9.
Step 2: Shares of each partner.
Hari = 3/9, Roy = 5/9, Prasad = 1/9.
Step 3: Adjustment after retirement.
Roy retires. His share = 5/9 is taken by Prasad.
So, new shares:
Hari = 3/9 = 1/3
Prasad = 1/9 + 5/9 = 6/9 = 2/3
Step 4: Ratio.
Hari : Prasad = 1 : 2.
Wait carefully! Actually above shows 1:2, but let us check again.
Hari = 3/9 = 1/3.
Prasad = 1/9 + 5/9 = 6/9 = 2/3.
Ratio Hari : Prasad = 1/3 : 2/3 = 1 : 2.
Correction. The correct ratio is 1 : 2.
Final Answer:
\[
\boxed{\text{1 : 2}}
\]