Đáp án đúng: A
Giải chi tiết:\(\begin{array}{l}10 + \left( {31 - x} \right) = 40\\\,\,\,\,\,\,\,\,\,\,\,\,\,\,31 - x = 40 - 10\\\,\,\,\,\,\,\,\,\,\,\,\,\,\,31 - x = 30\\\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,x = 31 - 30\\\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,x = 1\,\end{array}\)
Vậy \(x = 1\).
Chọn A.