\[\boxed{\text{515\ (515).}\text{\ }\text{Еуроки\ -\ ДЗ\ без\ мороки}}\]
\[\textbf{а)}\ 6 = 4 + 2 = 2^{2} + 2^{1}\ \]
\[\textbf{б)}\ 18 = 16 + 2 = 2^{4} + 2^{1}\]
\[\textbf{в)}\ 42 = 32 + 8 + 2 =\]
\[= 2^{5} + 2^{3} + 2\]