Adding and subtracting decimals is ordinary column arithmetic with one extra rule: line the numbers up by their decimal points, not by their last digits, filling the short one with zeros. To multiply by a whole number, ignore the point, multiply, then put the point back so the answer has the same number of decimal places as you started with. To divide, keep the point of the answer directly above the point in the number you are dividing. Multiplying or dividing by 10, 100 or 1000 is different again — the digits never change at all, only the point moves, one place for every zero.
Worked example
Problem Find the value of 3.45 + 2.7.
Line the two numbers up by their decimal points and write 2.7 as 2.70, so both have 2 decimal places.
Add the hundredths, then the tenths, then the ones, carrying as usual: 3.45 + 2.70.
The decimal point in the answer sits straight below the others, giving 6.15.
Tip Writing 2.7 as 2.70 does not change its value — it just gives the hundredths column something to add.