Twenty addition problems a sheet, the two numbers written one above the other with the decimal points aligned. Where one is shorter it is padded out to match - 4.6 above 12.35 prints as 4.60 - so the empty hundredths column is written in as a zero rather than left for the student to imagine.
Adding decimals is the same carrying as with whole numbers once the points are aligned: 2.85 + 1.40 is 285 + 140 with a point in it. Students who can do the second and not the first are usually losing the alignment, not the addition, which is why these sheets show it rather than hide it.
Line up the decimal points, fill the short number's missing places with zeros, then add column by column and carry as usual. Bring the point straight down into the answer.
Pad the shorter one with zeros on the right so both have the same number of places - 4.6 becomes 4.60 next to 12.35. These sheets print that padding for you, so the columns you add are the columns you see.
Yes. The arithmetic is done on whole numbers of thousandths, so there is no floating-point drift in the answer key.
Yes - the Subtracting Decimals page, which orders each pair so no answer is ever negative.