Be sure to line up the decimals when adding and subtracting decimals.
You can always add a decimal point to a whole number. For example, 5 = 5.00 or 5.0 or 5.000000. You can add as many zeros as you want because it doesn't change the value, it just makes it easier for us to compute.
Look for key words in the word problems to give you the clue as to add or subtract.