Answer: After 210 seconds they with next flash at the same time.
Step-by-step explanation:
Given: Ardem has two lights.
One flashes every 15 seconds the other flashes every 42 seconds.
If they started flashing at the same time, then the number of seconds after that they with next flash at the same time = LCM (15, 42) [LCM - Least common multiple]
Prime factorization of 15 = 3 x 5
Prime factorization of 42 = 2 x 3 x 7
Then, LCM (15, 42) = 2 x 3 x 5 x 7 = 210
i.e. After 210 seconds they with next flash at the same time.
