a, and b are positive integers less than or equal to 9 and b is twice the a. If a and b are assembled into the four-digit number abab, which of the following must be a factor of abab? Choose the correct answer: A) 5 B) 7 C) 9 D) 12 E) None of these
b = 2a is given thus possible numbers are 1212, 2424, 3636 and 4848. All are divisible by 12. Thus 12 must be a factor of abab   Answer: 12
<result>D</result>
a, and b are positive integers less than or equal to 9 and b is twice the a. If a and b are assembled into the four-digit number abab, which of the following must be a factor of abab?