S is a set of natural numbers from 1 to 100 whose smallest

Question

S is a set of natural numbers from 1 to 100 whose smallest prime factor is 5. What is the maximum number of elements of the set S? 
 

Answer Choices : 

(A) 7 
(B) 9 
(C) 30 
(D) 32 
(E) None of these

ricokevin · Answer

5 is one of the factors. So it's a multiple of 5.

So the numbers would be 5, 10, 15, 20, ...etc

Since 2 is also prime that is smaller than 5, exclude all even number from the list of numbers (10, 20, 30, etc)

Since 3 is also prime that is smaller than 5, exclude all those that are multiples of 3 (15, 45, 75, etc.)

You're left with 7 numbers (5, 25, 35, 55, 65, 85, 95).

So the answer is A.

Tuneman · Answer

Is there a good book, or a huge set of numerical number property questions?

techjanson · Answer

A
There are 20 numbers (ending in 5 and 0) who have a prime factor of 5. Out of these all numbers ending in 0 have 2 as the smallest prime factor so we discard them. Among remaining only 5, 25, 35, 55, 65, 85, 95 have 5 as smallest prime factor

kyatin · Answer

if you observe all the possible numbers are product of 5 and prime numbers that are equal to or greater than 5. 

It has to be that way for 2 and 3 to not figure out as one of the factors.

S is a set of natural numbers from 1 to 100 whose smallest

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