Step 1: Determine the old profit sharing ratio.
The old profit sharing ratio of the partners was:
\[
P : Q : R = 3 : 4 : 1
\]
Total parts $=3+4+1=8$
Thus,
\[
P=\frac{3}{8}, \quad Q=\frac{4}{8}, \quad R=\frac{1}{8}
\]
Step 2: Determine the share of R.
R’s share in the firm is:
\[
\frac{1}{8}
\]
R gives $\frac{1}{3}$ of his share to P.
\[
\text{Share received by P}=\frac{1}{3}\times\frac{1}{8}=\frac{1}{24}
\]
The remaining share goes to Q.
\[
\text{Share received by Q}=\frac{2}{3}\times\frac{1}{8}=\frac{2}{24}=\frac{1}{12}
\]
Step 3: Calculate the new shares of P and Q.
New share of P:
\[
\frac{3}{8}+\frac{1}{24}
\]
Convert into common denominator:
\[
\frac{3}{8}=\frac{9}{24}
\]
\[
P=\frac{9}{24}+\frac{1}{24}=\frac{10}{24}
\]
New share of Q:
\[
\frac{4}{8}+\frac{1}{12}
\]
Convert into common denominator:
\[
\frac{4}{8}=\frac{1}{2}=\frac{12}{24}, \quad \frac{1}{12}=\frac{2}{24}
\]
\[
Q=\frac{12}{24}+\frac{2}{24}=\frac{14}{24}
\]
Step 4: Determine the new profit sharing ratio.
\[
P:Q=\frac{10}{24}:\frac{14}{24}
\]
\[
P:Q=10:14
\]
Divide by $2$:
\[
P:Q=5:7
\]
Step 5: Conclusion.
Thus, after the retirement of R, the new profit sharing ratio of P and Q becomes $5:7$.
Final Answer: $5:7$