Answer: 495
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Step-by-step explanation
Find the prime factorization of each given value.
55 = 5*11
99 = 3*3*11
The GCF is 11 since it's the largest factor they both share.
LCM = (x*y)/GCF
LCM = (55*99)/11
LCM = 495
This formula only applies if you have two values. A slight work-around is needed if the set has more than two values.
You can confirm the answer by listing out multiples. I'll leave that for the student to do. Many various online calculators can find the LCM.