A father is currently twice as old as his son. Twenty years ago, he was four times as old. How old are they now?
Train A leaves the station at 12:00 noon travelling at 60 mph. Train B leaves the same station one hour later, at 1:00 pm, on the same track travelling 80 mph in the same direction. At what time does Train B catch up?
Alice can paint a fence in 4 hours working alone. Bob can paint the same fence in 6 hours working alone. How long does it take if they work together?