Two pipes P and Q, when opened alone can fill the tank in 20 and 30 hours respectively. If both pipes are opened together, then in how many hours will the tank be filled?
An observer on the top of a tree finds the angle of depression of a car moving towards the tree to be 30 degrees. After 3 minutes, this angle becomes 60 degrees. After how much more time will the car reach the tree?
4 men and 5 women can complete a work in 15 days, whereas 9 men and 6 women can do it in 10 days. To complete the same work in 7 days, how many women should assist 4 men?