Adding and subtracting multi-digit numbers

Line up the places

The standard algorithm works because of place value: you add ones to ones, tens to tens, hundreds to hundreds. So the first rule is to line up the digits on the right, ones under ones.

Adding with regrouping

Subtracting with regrouping

If a top digit is smaller than the one below it, regroup (borrow) from the next place: \(1\) ten becomes \(10\) ones.

Check your work

Decimals: line up the decimal points

Mental math

Practice