Math & Science ⇒ Distributive property alignment worksheet

[robtweet2]

I'm trying to create a worksheet for 3rd graders that has an example of the distributive property, and I'm having trouble aligning the columns. Here's what I have:

Code: Select all

\begin{alignat*}{4}
7 &\times 142 & &  &= \\
7 &\times (100 &+ 40 &+ 2) &= \\
& (100 \times 7) &+ (40 \times 7) &+ (2 \times 7) &=  \\
& 700 &+ 280 &+ 14 &=  
\end{alignat*}

which produces the attached image (without the red and blue lines). I want the addition signs to line up where the blue line is located. I'd be grateful for any help.

[robtweet2]

I think I found an answer, but I don't know if it is the best solution. I believe the problem with using alignat is that it alternates between right and left alignment for each column. To get all the addition signs to align, I used an array, centered all the columns, and made a columns for the operands and operators. Here is my solution:

Code: Select all

$
\begin{array}{cccccccc}
7 & \times & 142 & & & & & = \\
7 & \times & (100 & + & 40 & + & 2) & = \\
 & & (100 \times 7) & + & (40 \times 7) & + & (2 \times 7) & = \\
 & & 700 & + & 280 & + & 14 & = \\
\end{array}
$

This created the following: