Question

Three lighthouses on three nearby islands flash their lights every evening. On one island, the light flashes every 45 seconds. The lighthouse on the second island has a light that flashes every 2 minutes. The third lighthouse flashes a light every 1 1/4 minute. Suppose all the lights flash together at 8:35 P.M. At what time will all three lights flash together again?

Answer 1

Lt1 = 45; Lt2 = 120; Lt3 = 75

:

Prime factor each:

45: 3,3,5,

120: 2,2,2,3,5

75: 3,5,5

:

Lcm of all three: 2,2,2,3,3,5,5 Multiply this: 1800 secs they will all Flash

:

1800/60 = 30 min: At 9:05Pm this should occur