Asked: If k is a positive integer, which of the following must be divisible by 24?
(A) (k – 4)(k)(k + 3)(k + 7)
(B) (k – 4)(k – 2)(k + 3)(k + 5)
(k-4)(k-2) is divisible by 2*4 = 8
(k-4)(k-3)(k-2) is divisible by 3.
If (k-3) is divisible by 3, then (k-3)+6 = (k+3) is also divisible by 3.
Otherwise one of (k-4) & (k-2) is divisible by 3.
(k – 4)(k – 2)(k + 3)(k + 5) is divisible by 8*3 = 24
(C) (k – 2)(k + 3)(k + 5)(k + 6)
(D) (k + 1)(k + 3)(k + 5)(k + 7)
(E) (k – 3)(k + 1)(k + 4)(k + 6)
IMO B
Second approachLet us put k=7 & k=8
(A) (k – 4)(k)(k + 3)(k + 7)
If k=7; (k – 4)(k)(k + 3)(k + 7) = 3*7*10*14; Divisible by 3*2*2 = 12 but no by 24
Incorrect
(B) (k – 4)(k – 2)(k + 3)(k + 5)
If k=7; (k – 4)(k – 2)(k + 3)(k + 5) = 3*5*10*12; Divisible by 3*2*4 = 24
If k=8; (k – 4)(k – 2)(k + 3)(k + 5) = 4*6*11*13; Divisible by 4*6 = 24
(C) (k – 2)(k + 3)(k + 5)(k + 6)
If k=7; (k – 2)(k + 3)(k + 5)(k + 6) = 5*10*12*13; Divisible by 2*12 = 24
If k=8; (k – 2)(k + 3)(k + 5)(k + 6) = 6*11*13*15; Divisible by 6
Incorrect
(D) (k + 1)(k + 3)(k + 5)(k + 7)
If k=7; (k + 1)(k + 3)(k + 5)(k + 7) = 8*10*12*14; Divisible by 2*12 = 24
If k=8; (k + 1)(k + 3)(k + 5)(k + 7) = 9*11*13*15; Divisible by 3
Incorrect
(E) (k – 3)(k + 1)(k + 4)(k + 6)
If k=7; (k – 3)(k + 1)(k + 4)(k + 6) = 4*8*11*13; Divisible by 8
Incorrect
IMO B