Adding and Subtracting Fractions

La méthode, étape par étape

1. 1

   Find the lowest common denominator

   2/3 + 1/4 → LCM of 3 and 4 is 12

   The lowest common denominator is the LCM of the denominators. Any common multiple works — 24 would give a correct answer — but the lowest one keeps the numbers small and usually removes the need to simplify at the end.

2. 2

   Rewrite each fraction over that denominator

   2/3 = 8/12 · 1/4 = 3/12

   Multiply top and bottom by whatever turns the denominator into 12. This is equivalence doing the work: neither fraction has changed value, they have only been rewritten so the pieces are the same size and can be counted together.

3. 3

   Add the numerators only

   8/12 + 3/12 = 11/12

   The denominator says what size the pieces are; adding eight twelfths to three twelfths gives eleven twelfths, not eleven twenty-fourths. Adding denominators is the single most common error in the topic and it comes from treating the fraction as two independent numbers.

4. 4

   Handle mixed numbers by separating or converting

   2¾ + 1½ → wholes 2+1 = 3; parts ¾ + ½ = 5/4 = 1¼ → 4¼

   Either add the whole parts and the fractional parts separately and recombine, or convert both to improper fractions first. Both are correct; the separate method is faster and the improper method is safer when subtraction requires exchanging from the whole.

5. 5

   Simplify and sanity-check

   11/12 — already lowest terms. Both fractions were under 1, so a sum under 2 is sensible ✓

   Check the size before accepting the answer: two fractions each less than one cannot sum to more than two. This catches the added-denominators error instantly, because that error produces an answer far too small.

Why the denominators have to match

A denominator is not a number being operated on; it is a unit. Two thirds means two pieces of the size called 'third', and one quarter means one piece of the size called 'quarter'. Adding them directly is like adding two metres to one foot and writing three — the count is meaningless until the units agree.

Framing it this way makes the whole method obvious rather than procedural. You are not following a rule about common denominators; you are converting to a common unit so the counting means something, exactly as you would with any other measurement.

• Denominator = the size of the piece
• Numerator = how many pieces
• Only the counts add; the size stays fixed
• Lowest common denominator = LCM of the denominators

The error that defines this topic

Writing 1/2 + 1/3 = 2/5 is so common it has a name in the research literature. It comes from treating the fraction as a pair of whole numbers and operating on each, which is exactly what multiplication of fractions does allow — and that inconsistency is genuinely confusing rather than careless.

The quickest cure is a size check rather than a rule. One half is already more than 2/5, so a sum of two positive fractions cannot be smaller than either of them. Students who habitually ask 'is this answer a sensible size?' stop making this error without needing to remember which operation permits what.

Where subtraction gets awkward

Subtracting mixed numbers is the hardest case, because the fractional part of the first number may be smaller than the second's. In 3¼ − 1¾, the quarter cannot lose three quarters, so a whole must be exchanged into four quarters first — the same exchange as column subtraction, in a different costume.

Converting both to improper fractions avoids the exchange entirely and is the safer route for most students: 13/4 − 7/4 = 6/4 = 1½. It handles more digits and fewer decisions, which is usually the better trade under exam pressure.

How we teach adding fractions

We teach the denominator as a unit from the first lesson and keep using that language. Students who describe 2/3 as 'two thirds' rather than 'two over three' rarely add denominators, because the phrase itself makes the error sound wrong.

We also require a size check on every answer. It costs three seconds, it catches the topic's defining mistake, and it builds the habit that later catches sign errors and misplaced decimal points elsewhere.