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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 ...

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:

\begin{alignat*}{4}
7 &\times 142 & & &= \\
7 &\times (100 &+ 40 &+ 2) &= \\
& (100 \times 7) &+ (40 \times 7) &+ (2 \times 7 ...