Three builders are laying a brick wall.
Fred can lay 1000 bricks in three hours, Jimmy can lay 1000 bricks in two hours and Kev can lay 1000 bricks in six hours.
How long will it take Fred, Jimmy and Kev to lay 1000 bricks if they all work together?
Answer: 1 hour.
How many bricks are laid in six hours?
In six hours Kev lays 1000 bricks, Fred lays 2000 bricks, and Jimmy lays 3000 bricks.
So in six hours we have a total of 6000 bricks laid.
Which means that it will take one hour to lay 1000 bricks. QED.