nick1816 wrote:
How many 3 digit numbers have exactly 3 factors?
A. 15
B. 22
C. 21
D. 23
E. 7
My Approach.
You should realize only a perfect square has odd number of factors.
16 has 1,2,4,8,16. 25 has 1,5,25.
Think of the least 3 digit perfect square. 121. Hmm 1,11,121. Works.
12 won't work but 13 will. 14 won't work but 17 does. So a pattern should click now.
11,13,17,19,23,29. I actually stopped here and thought 6. but there is no option of 6 (Thank God)
so thought wait 31 works too.
Hence E.
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