If x and y are integers, what is the least positive number of 24x+21y?  Choose the correct answer: A) 3 B) 5 C) 6 D) 8 E) 9

Rationale: "24 x + 21 y = 3 ( 8 x + 7 y ) which will be a minimum positive number when 8 x + 7 y = 1 . 8 ( 1 ) + 7 ( - 1 ) = 1 then 3 ( 8 x + 7 y ) can have a minimum positive value of 3 . the answer is a ."
Answer: A