Question

A lighthouse flashes a red light every 6 minutes and a green light every 9 minutes. At 7:00 pm the lighthouse flashes red and green lights at the same time. Name the next three times that the lighthouse will flash the red and green lights at the same time.

Answer 1

Answer:
To calculate the time at which they flash together we need to find the least common multiple of 3 and 8.

Multiples of 3 =

3 x 1 = 3​

3 x 2 = 6​

3 x 3 = 9​

3 x 4 = 12​

3 x 5 = 15​

3 x 6 = 18​

3 x 7 = 21​

3 x 8 = 24​

3 x 9 =27​

3 x 10 = 30

Multiples of 8 =

8 x 1 = 8​

8 x 2 = 16​

8 x 3 = 24

8 x 4 = 32

8 x 5 = 40​

8 x 6 = 48

8 x 7 = 56​

8 x 8 = 64​

8 x 9 =72​

8 x 10 = 80

So LCM of 3 and 8 = 24

So both the lights will flash next at 7:24

Explanation: