A and B are partners in a firm, their capital are ₹ 10,000 and ₹ 20,000 and undistributed reserves ₹…

Based on C's capital of ₹ 20,000 for a $\frac{1}{4}$ share, the implied total capital of the new firm is $₹\,20{,}000 \times 4 = ₹\,80{,}000$. Subtracting the combined actual capital of all partners including undistributed reserves ($₹\,10{,}000 + ₹\,20{,}000 + ₹\,10{,}000 + ₹\,20{,}000 = ₹\,60{,}000$) gives hidden goodwill of $₹\,80{,}000 - ₹\,60{,}000 = ₹\,20{,}000$.