can you explain about the interest calculation in illustration 6 in the chapter accounts from incomplete records ?

Loan by Mr. Anish is outanding in the books of business as on Mar 31,2012 is Rs 1,000 (outstanding for 6 months) out of which Rs 500 had been paid on Oct 01,2012. Hence, Rs 500 (i.e. 1,000-500,outstanding for 6 months). 
Therefore, the Interest on Loan is calculated as follows:
Interest on Rs 1,000 (loan outstanding as on Mar 31,2012 till Oct 01,2012) = Rs 20 (1,000*4/100*6/12)
Interest on Rs 500 (loan outstanding as on Mar 31,2013) = Rs 10 (5000*4/100*6/12).