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?
You have 10 litres of a 30% acid solution. How many litres of pure water do you need to add to dilute it to a 20% acid solution?