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GB-09-04-001 · Repeated events

Two lights flash together. One flashes every 9 seconds and the other every 10 seconds. After how many seconds will they next flash together?

LCM in Context (Repeating Events) Booklet p. 42
Method
1) Simultaneous repeat time = Lowest Common Multiple (LCM) of the two intervals
2) Use prime factors / Venn diagram to find LCM
Worked Example (GB-09-04-001)
1)
Prime factorise each number: 9=3×39=3\times3, 10=2×510=2\times5.
2)
9103 · 3none2 · 5
3)
Place each shared prime in the intersection once. Multiply all factors in the diagram: LCM = 90.
Ans = 90 seconds