गोरी और गोपाल अकेले किसी कार्य को क्रमशः 9 और 21 दिनों में पूरा कर सकते हैं। यदि वे दोनों गोपाल से शुरू करते हुए वैकल्पिक दिनों (alternate days) में कार्य करते हैं, तो कार्य कितने दिनों में पूरा होगा?

If P and Q together can complete a piece of work in 15 days and P alone can do it in 45 days, in how many days can Q alone complete that work?

A can lay a railway track between two stations in 16 days and B can do the same job in 12 days. With the help of C, they finished the job in 4 days. In how many days can C alone do the job?