Replace [tex]a_{n}[/tex] with 27, solve for [tex]a_{n-1}[/tex], and see if the result is a whole number.
27 = 2a ⇒ [tex]\frac{27}{2}[/tex] = a not a whole number NO
27 = 2a + 1 ⇒ 26 = 2a ⇒ 13 = a 13 is a whole number YES
27 = 3a ⇒ 9 = a 9 is a whole number YES
27 = 3a + 6 ⇒ 21 = 3a ⇒ 7 = a 7 is a whole number YES
27 = 3a + 1 ⇒ 26 = 3a ⇒ [tex]\frac{26}{3}[/tex] = a not a whole number NO
Answer: B, C, D
