If a three digit number ‘abc’ has 2 factors (where a, b, c are digits), how many factors does the 6-digit number ‘abc’ have?
16
24
18
30
Solution
To start with ‘abcabc’ = ‘abc’ * 1001 or abc * 7 * 11 * 13 (This is a
critical idea to remember).
‘abc’ has only two factors. Or, ‘abc’ has to be prime. Only a prime number
can have exactly two factors. (This is in fact the definition of a prime
number)
So, ‘abcabc’ is a number like 101101 or 103103.
’abcabc’ can be broken as ‘abc’ * 7 * 11 * 13. Or, a p * 7 * 11 * 13 where
p is a prime.
As we have already seen, any number of the form paqbrc will have (a + 1) (b
+ 1)(c + 1) factors, where p, q, r are prime.
So, p * 7 * 11 * 13 will have = (1 + 1) * (1 + 1) * (1 + 1) * (1 + 1) = 16
factors