The correct option is B 40 seconds
To find when the three lights will flash together again, we need to find the least common multiple (LCM) of the numbers 5, 8 and 10.
Multiples of 5 = 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, ...
Multiples of 8 = 8, 16, 24, 32, 40, 48, 56, 64, 72, 80, ...
Multiples of 10 = 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, ...
40 is the smallest multiple common to 5, 8 and 10.
∴ The three lights will flash together again after 40 seconds.