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

Your answer seems reasonable.
Find out if you're right!