\(B=1+2+2^2+2^3+...+2^{2015}\)
\(B=\left(1+2+2^2\right)+\left(2^3+2^4+2^5\right)+...+\left(2^{2013}+2^{2014}+2^{2015}\right)\)
\(B=\left(1+2+2^2\right)+2^3\left(1+2+2^2\right)+...+2^{2013}\left(1+2+2^2\right)\)
\(B=7+2^3.7+... +2^{2013}.7\)
\(B=7\left(1+2^3+...+2^{2013}\right)⋮7\)