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Subtracting Decimals

Overview

Precision matters.

What is it?

Finding the difference between decimal values.

History

Popularized by Simon Stevin in the 16th century to simplify interest calculations.

Key Idea

Line up the Dots! Give them ghost zeros.

Practice This Topic

Concept Guide

Plain English: It is exactly like regular subtraction, but you MUST line up the decimal points. If one number is shorter, fill the empty space with a zero.

Real-world example: Money. $20 - $14.50. You need to write 20.00 to make it work.

How to do it

  1. Line up the decimal points vertically.
  2. Fill any empty spots with '0'.
  3. Borrow and subtract normally.
  4. Drop the decimal point straight down.
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Common Pitfall

Subtracting from air. You can't do 'nothing minus 5'. You must write the placeholder zero.

Word Problem
"Lap 1 was 55.4s. Lap 2 was 54.9s. How much faster was Lap 2?"
Reasoning: 55.4 - 54.9. Borrow from 5. 14-9=5. Answer: 0.5 seconds.

Practice Examples

Ghost Zeros
5.0 - 3.5 ----- 1.5
Added .0 to the 5.