Adding and subtracting decimals uses the exact same steps as whole numbers — you just need to line up the decimal points first. Get that one step right, and the rest is familiar.
To add or subtract decimals, stack them so the decimal points fall directly under one another — just like lining up the ones place in whole-number addition. If the numbers don't have the same number of decimal places, add zeros to the end of the shorter one. This doesn't change its value, it just fills in the missing columns.
Notice 2.7 became 2.70 — the dim digit is the zero we added just to line up the columns. It doesn't change the amount, it just gives every column a partner to add with.
Each column has to represent the same place value — tenths under tenths, hundredths under hundredths — or the addition doesn't make sense. It's the same reason you'd never add a tens digit to a ones digit in whole-number addition.
Once everything is lined up, add each column from right to left — exactly like whole-number addition. If a column adds up to 10 or more, carry the 1 into the next column to the left, same as always. Bring the decimal point straight down into your answer.
Hundredths: 5 + 0 = 5. Tenths: 4 + 7 = 11 — write the 1, carry a 1 into the ones column (shown in teal above that column). Ones: 3 + 2 + 1 (carried) = 6. Bring the decimal point straight down: 3.45 + 2.70 = 6.15.
Here's the problem written out normally. Line up the decimal points, then solve column by column, right to left.
Subtracting works the same way — line up the decimal points, pad with zeros if the numbers have a different number of decimal places, then subtract column by column from right to left. If a column doesn't have enough to subtract from, borrow from the column to its left, just like whole-number subtraction.
Hundredths: 0 − 7 isn't enough on its own, so we borrow from the tenths column — shown in teal, that's really 10 − 7 = 3, not 0 − 7. Tenths: after lending 1, that column is really 11 − 4 = 7. Ones: after lending 1, 5 − 1 = 4. Bring the decimal point straight down: 5.20 − 1.47 = 3.73.
Borrowing works exactly the same whether or not a decimal point is in the way — borrowing from the ones place into the tenths place is no different from borrowing from the tens place into the ones place. Whenever a column borrows, we'll show the borrowed ten in teal so it's clear you're really working with 10, 11, or more — not just the lone digit that was sitting there before.
This matters even more when subtracting from a whole number. A whole number like 5 is really 5.00 — with invisible zeros after the decimal point, just waiting to be borrowed from.
Think of 5 as 5.00. Hundredths: 0 − 7 needs to borrow — 10 − 7 = 3. Tenths: 0 also has nothing to give, so it borrows too, ending up as 9 − 4 = 5. Ones: 5 lends 1 away and becomes 4 − 1 = 3. 5 − 1.47 = 3.53.
Here's the problem written out normally. Line up the decimal points, then solve column by column, right to left.