Note: In this question, as the balances of both Partner' Capital Account and of Partner' Current Account are mentioned, so it has been assumed that the capital of the partners is fixed.
As we know, when the capital of the partners is fixed, drawings and interest on capital does not affect the capital balances of the partners. Rather, it would affect their current account balances. Therefore, in this case, capital at the beginning (i.e. opening capital) and capital at the end (i.e. closing capital) of the year would remain same. Thus, the interest on capital is calculated on fixed capital balances (given in the Balance Sheet of the question).
