Step 1: Determine total capital of existing partners.
Capital of Alex = Rs. 1,20,000
Capital of Benn = Rs. 80,000
Capital of Cole = Rs. 1,00,000
\[
\text{Total capital of old partners} = 1,20,000 + 80,000 + 1,00,000 = 3,00,000
\]
Step 2: Determine Dona's share in profits.
Dona is admitted for \( \frac{1}{5} \) share in future profits.
Therefore, the old partners together will share:
\[
1 - \frac{1}{5} = \frac{4}{5}
\]
Step 3: Determine total capital of the new firm.
The capital of Rs. 3,00,000 corresponds to \( \frac{4}{5} \) of the total capital.
\[
\text{Total capital of the firm} =
\frac{3,00,000}{4} \times 5
\]
\[
= 75,000 \times 5
= 3,75,000
\]
Step 4: Calculate Dona's capital contribution.
Dona's share = \( \frac{1}{5} \) of total capital.
\[
\text{Dona's capital} =
\frac{1}{5} \times 3,75,000
\]
\[
= 75,000
\]
Step 5: Conclusion.
Therefore, Dona must bring Rs. 75,000 as capital to maintain the proportion of the profit-sharing ratio.
Final Answer: Rs. 75,000.