Question 23
Four lamps numbered 1, 2, 3 and 4 repeatedly alternate between green and red. Lamp number $n$ stays green for $n$ seconds, then red for $n$ seconds, and repeats. For example, lamp 1 is green for 1 second then red for 1 second; lamp 2 is green for 2 seconds then red for 2 seconds. All four start green when switched on. During the first 50 seconds, for how many seconds are all four green together, including the initial green interval?
