Can someone explain?
An integer greater than 1 that is not prime is called composite. If the two digit integer n is greater than 20, is n composite?
1) The tens digit of n is a factor of the units digit of n. In other words, the units digit is a nonzero multiple of the tens digit.
2) The tens digit of n is 2
(1) Let 10>m>1 be the tens digit of n, then the units digit of n is km for some positive integer k<10
Thus n=10m+km=(10+k)m. Clearly n is composite (i.e non-prime) SUFF
(2) n could be 23 (prime) or 24 (composite) among other possibilities N SUFF
I get A
We could also mentally check (1) by examining the possilbities:
22 24 ..28 all composite
33 36 39
55 66 77 88 99