Two lights flash every 12 seconds and every 18 seconds. They flash together at time 0. After how many seconds do they next flash together?
Answer and explanation
Correct answer: C. 36 seconds
- Step 1: They coincide at common multiples of 12 and 18; the first one is the least common multiple.
- Step 2: 12 = 4 x 3 and 18 = 2 x 9, so the LCM is 4 x 9 = 36.
Answer
36 seconds.
Why not the others: 6 is the common divisor, 216 is a common multiple but not the least, and 72 is a later coincidence.
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