If x and y are integers, what is the least positive number of 24x+18y?  Answers
A) 3 B) 6 C) 9 D) 12 E) 15

Rationale: "24 x + 21 y = 6 ( 4 x + 3 y ) which will be a minimum positive number when 4 x + 3 y = 1 . 4 ( 1 ) + 3 ( - 1 ) = 1 then 6 ( 4 x + 3 y ) can have a minimum positive value of 6 . the answer is b ."
Answer: B