Lighthouses
Year 4 Version
On the coast there are three lighthouses
The first light shines for 3 seconds then it is off for three seconds. The second light shines for 4 seconds then it is off for 4 seconds. The third light shines for 5 seconds then it is off for 5 seconds
All three lights have just come on together.
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When is the first time that all three of the lights will be off together?
answers
Make an on/off table for the three lighthouses
lighthouse A 1 1 1 0 0 0 1 1 1 0 0 0
lighthouse B 1 1 1 1 0 0 0 0 1 1 1 1
lighthouse C 1 1 1 1 1 0 0 0 0 0 1 1
they will be off together in the 6th second
To find when they all come on together, find the
lowest common multiple of their time intervals 6, 8 and 10
LCM 6 6 12 18 24 30 36 42 48 54 60 66
72 78 84 90 96 102 108 114 120 126 132
LCM 8 8 16 24 32 40 48 56 64 72 80 96
104 112 120 128 136 144
LCM 10 10 20 30 40 50 60 70 80 90 100 110
120 130 140 |