It's Equal To One Of The Others?

Algebra Level 1

\[ \begin{array} { c c c c c c c c c c c c } a & + & b & + & c & & & = & 3 \\ a & + & b & & & + & d & = & 4 \\ a & & & + & c & + & d & = & 5\\ & & b & + & c & + & d & = & 6 \\ a & + & b & + & c & + & d & = & ? \\ \end{array} \]