Gọi số học sinh khối 6 là x
Theo đề, ta có: \(\left\{{}\begin{matrix}x+1\in BC\left(2;3;4;5;6\right)\\x\in B\left(7\right)\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}x+1\in B\left(60\right)\\x\in B\left(7\right)\end{matrix}\right.\)
\(\Leftrightarrow\left\{{}\begin{matrix}x+1\in\left\{60;120;180;240;300;...\right\}\\x\in B\left(7\right)\end{matrix}\right.\Leftrightarrow x=119\)