Answer: d) -5, -40, -640
<u>Step-by-step explanation:</u>

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Answer:
30 seconds
Step-by-step explanation:
Find the least common multiple of 6 and 15.
First, write the prime factorizations.
6 = 2 × 3
15 = 3 × 5
The LCM is the product of each distinct factor.
LCM = 2 × 3 × 5
LCM = 30
They flash together at the same time every 30 seconds.