Decimal Operations

Pre-Algebra · Axiom Academy

All four operations with decimals — and the one idea behind each rule: keep digits with the same place value together. 1. Addition & Subtraction: Line Up the Points To add or subtract decimals, write one number above the other so the decimal points sit in the same vertical column . That forces tenths over tenths and hundredths over hundredths — exactly the digits that belong together. Think of money: you stack dollars over dollars and cents over cents. Align the points, then add column by column Points stacked — tenths over tenths. Points not aligned — values misread. 2. Multiplication: Count the Places For multiplication you do not line up the points. Instead, ignore the decimals and multiply the numbers as whole numbers. Then count the total number of decimal places in the two factors, and place the point that many digits from the right of your answer. Multiply whole, then place the point by counting places A decimal place means a factor of ten in the denominator: and . Multiplying multiplies the bottoms too — — so two places in plus one more gives the places out. It is the same as . 3. Division: Make the Divisor Whole To divide by a decimal, first turn the divisor into a whole number by sliding its decimal point to the right. Then slide the point in the dividend the same number of places . Now you are dividing by a whole number, which is the case you already know. Slide both points the same number of steps

This is the written version of the interactive lesson above. See the full Pre-Algebra course.