Answer:
C
Explanation:
find the LCM of 10 and 15
multiples of 10 are 10, 20, 30, 40, .....
multiples of 15 are 15, 30, 45, 60, ....
the LCM of 10 and 15 is 30
they will flash together again after 30 seconds
1.6m questions
2.0m answers