The partnership deed provides for interest on capital at 8\% p.a., and the partners' fixed capitals are as follows:
\[
{Ridhima's Capital: } Rs.1,50,000, \quad {Kavita's Capital: } Rs.2,00,000.
\]
Step 1: Calculate the total interest on capital entitlement:
\[
{Ridhima's Interest: } Rs.1,50,000 \times \frac{8}{100} = Rs.12,000.
\]
\[
{Kavita's Interest: } Rs.2,00,000 \times \frac{8}{100} = Rs.16,000.
\]
Total entitlement for interest on capital:
\[
Rs.12,000 + Rs.16,000 = Rs.28,000.
\]
Step 2: Allocate available profit proportionately:
The total profit of the firm is Rs.21,000, which is insufficient to meet the total entitlement of Rs.28,000. The available profit is distributed in the ratio of the interest entitlement:
\[
{Ratio of Interest Entitlement: } 12 : 16 = 3 : 4.
\]
\[
{Ridhima's Share: } Rs.21,000 \times \frac{3}{7} = Rs.9,000.
\]
\[
{Kavita's Share: } Rs.21,000 \times \frac{4}{7} = Rs.12,000.
\]
Conclusion:
The amount of interest on capital credited to Ridhima and Kavita's capital accounts is \( Rs.9,000 { and } Rs.12,000 \), respectively.