Subtraction within 100

Take away the tens and the ones. If the ones are too small, borrow a ten. 42 − 17 = 25.

Subtract by place value. When the ones digit of the top number is smaller, regroup one ten as ten ones (borrowing).

Columnar subtraction with regrouping: borrow 1 from the tens column (worth 10 ones) when the ones column is insufficient. 42 − 17: ones 2−7 needs borrowing → 12−7=5, tens 3−1=2, giving 25.