You should have come across such questions when you learnt factors & multiples in primary school. The method of solving is similar. The difference now is that, at higher level, the numbers are bigger.

The red light flash every 18 seconds, means that every multiple of 18 seconds, the red light with flash. Similarly, the blue light flashes every multiple of 28 seconds, and the green light flashes every multiple of 40 secs. The 3 lights will flash together when you have a common multiple of 18, 28 and 40.

For sec 1, you should be learning the "ladder" method of find LCM, which is as follows …

2 | 18, 28, 40
2 | 9, 14, 20
2 | 9, 7, 10
3 | 9, 7, 5
3 | 3, 7, 5
5 | 1, 7, 5
7 | 1, 7, 1
__| 1, 1, 1

LCM is 2x2x2x3x3x5x7 = 2520

2520 secs = 42 mins

the 3 lights will flash together again
at 08 42.

Thank you so much!