what number should replace the question mark ? 11 , 15 , 16 , 12 , 21 , 9 , - - - ?
"answer : e 11 , 15 , 16 , 12 , 21 , 9 , - - - ? there are two alternate sequences : + 5 and - 3 ."
add(add(add(const_2.0, 11), 11), 11)