Tìm \(x\) biết: \(\left( {x + 3} \right) + \left( {x + 7} \right) + \left( {x + 11} \right) + \cdots + \left( {x + 79} \right) = 860.\)

Đáp án đúng là: B                              

\(\left( {x + 3} \right) + \left( {x + 7} \right) + \left( {x + 11} \right) + \cdots + \left( {x + 79} \right) = 860.\)

\( \Leftrightarrow \left( {x + 3 + x + 79} \right) + \left( {x + 7x + 75} \right) + \cdots + \left( {x + 79 + x + 3} \right) = 1720\)

\( \Leftrightarrow 20\left( {x + 3 + x + 79} \right) = 1720\)

\( \Leftrightarrow 20\left( {2x + 82} \right) = 1720 \Leftrightarrow x = 2.\)

Vậy \(x = 2.\)