A, B and C and do a piece of work in 15 days, 10 days and 30 days respectively. A helped by B and C every third day. In how many days will the work be completed - A, B और C किसी काम को क्रमशः 15 दिन, 10 दिन और 30 दिनों में पूरा कर सकते हैं। A की सहायता B और C द्वारा प्रत्येक तीसरे दिन की जाती है तो वह कार्य कितने दिनों पूरा हो जायेगा?

A's work rate = 1/15, B's = 1/10, C's = 1/30. On days when A works alone, he does 1/15. On every third day, A+B+C work together doing 1/15 + 1/10 + 1/30 = 2/30 + 3/30 + 1/30 = 6/30 = 1/5. In 9 days: A works alone on 6 days (6×1/15 = 2/5) and all three work on 3 days (3×1/5 = 3/5). Total work = 2/5 + 3/5 = 1, so work completes in 9 days. Therefore, option B is correct.