Question 166271
Three lighthouses on three nearby islands flash their lights every evening 
beginning at 8:35pm. On one island, the ligh 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 minutes. 
At what time will all three lights flash together again?
:
Convert 2 min to seconds and 1.25 min to seconds. 
:
So we have:
:
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