The age of Dr. Pandey is four times the age of his son. After 10 years , the age of Dr. Pandey will be twice the age of his son . The present age of Dr. Pandey's son is
Raghu's tanker can fill a cistern in 4 hours. After half the cistern is filled, three more similar tankers are opened. What is the total time taken to fillthe cistern completely?