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Set A is formed by selecting some of the numbers from the first 100 natural numbers such that the HCF of any two numbers in the set is 5. What is the maximum number of elements that A can have?

a)7

b)8

c)9

d)10

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A={5,10,15,25,35,55,65,85,95}

consider product of a prime number and 5 which will give you the set and since range is 1 to 100 we will have 9 elements. (including 5 also)

so answer is C.

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