350,039 views
37 votes
37 votes
A light flashes every 15 minutes.

A second light flashes every 18 minutes.
Both lights flash together at 2 a.m.
What will be the time when they next flash together?​

User Regent
by
2.9k points

2 Answers

11 votes
11 votes

Explanation:

the smallest common multiple of 15 and 18 gives us the answer.

the get the smallest common multiple we typically use prime factoring :

15 ÷ 2 no

15 ÷ 3 = 5

5 ÷ 3 no

5 ÷ 5 = 1 finish

18 ÷ 2 = 9

9 ÷ 2 no

9 ÷ 3 = 3

3 ÷ 3 = 1 finish

15 = 3×5

18 = 2×3×3

the smallest common multiple is the combination of the longest streaks per prime factor.

1 time 2, 2 times 3, 1 time 5

2×3×3×5 = 90

so, every 90 minutes they flash together.

that means the next time after 2 am would be

2 am + 90 minutes = 3:30 am

User Munazza
by
2.5k points
12 votes
12 votes

Answer:

3:30 a.m.

Explanation:

LCM of 15 and 18 = 90.

90 minutes after 2 a.m. = 3:30 a.m.

User Anidhya Ahuja
by
3.2k points