When the date/dates of drawings is/are not given, then we assume that the amount of drawings is withdrawn evenly throughout the year and charge interest on totak drawubgs for a period of six months. So in the given question, amounts withdrawn by each partner is given but when and how much was withdrawn is not given, hence will will charge interest for six months only.
Interest on Drawings (A) (6000 x 6/100 x 6/12) = Rs.180
Interest on Drawings (B) 4000 x 6/100 x 6/12) = Rs.120