Multiply Mixed Numbers | Fifth Grade
Complete Notes & Formulas
1. Estimate Products of Mixed Numbers
Definition: Estimate the product of mixed numbers by rounding each mixed number to the nearest whole number before multiplying.
📝 Rounding Rules:
- If fraction < 1/2 → Round DOWN to whole number
- If fraction ≥ 1/2 → Round UP to next whole number
- After rounding, multiply the whole numbers
✏️ Example: Estimate 4 2/3 × 5 1/4
Step 1: Round 4 2/3 → 2/3 > 1/2, so round to 5
Step 2: Round 5 1/4 → 1/4 < 1/2, so round to 5
Step 3: Multiply: 5 × 5 = 25
Estimate: About 25
2. Multiply a Mixed Number by a Whole Number
Definition: Convert the mixed number to an improper fraction, write the whole number as a fraction, then multiply.
📐 Formula:
W a/b × n = [(W × b + a)/b] × n/1
📝 Steps:
- Convert mixed number to improper fraction: (W × b + a)/b
- Write whole number as fraction: n = n/1
- Multiply numerators and denominators
- Simplify and convert back to mixed number
✏️ Example: 2 1/3 × 4
Convert: 2 1/3 = (2 × 3 + 1)/3 = 7/3
Multiply: 7/3 × 4/1 = 28/3
Convert: 28/3 = 9 1/3
Answer: 9 1/3
3. Multiply a Mixed Number by a Fraction
Definition: Convert the mixed number to an improper fraction, then multiply it by the given fraction.
📐 Formula:
W a/b × c/d = [(W × b + a)/b] × c/d = [(W × b + a) × c]/(b × d)
✏️ Example: 3 1/2 × 2/5
Convert: 3 1/2 = 7/2
Multiply: 7/2 × 2/5 = 14/10
Simplify: 14/10 = 7/5 = 1 2/5
Answer: 1 2/5
4. Multiply Two Mixed Numbers
Definition: Convert both mixed numbers to improper fractions, multiply them, then convert the result back to a mixed number.
📐 General Formula:
W₁ a/b × W₂ c/d = [(W₁×b+a)/b] × [(W₂×d+c)/d]
📝 Steps:
- Convert first mixed number to improper fraction
- Convert second mixed number to improper fraction
- Multiply the two improper fractions
- Simplify the result
- Convert to mixed number if improper
✏️ Example: 2 3/4 × 1 1/2
Convert first: 2 3/4 = (2 × 4 + 3)/4 = 11/4
Convert second: 1 1/2 = (1 × 2 + 1)/2 = 3/2
Multiply: 11/4 × 3/2 = 33/8
Convert: 33/8 = 4 1/8
Answer: 4 1/8
5. Multiply Mixed Numbers, Fractions, and Whole Numbers
Definition: When multiplying a combination of mixed numbers, fractions, and whole numbers, convert all to improper fractions first.
📝 Conversion Rules:
- Mixed Number: Convert to improper fraction (W × b + a)/b
- Whole Number: Write as fraction n/1
- Fraction: Keep as is
- Then: Multiply all numerators, multiply all denominators
✏️ Example: 2 1/2 × 3 × 1/4
Convert mixed: 2 1/2 = 5/2
Convert whole: 3 = 3/1
Fraction stays: 1/4
Multiply: 5/2 × 3/1 × 1/4 = 15/8
Convert: 15/8 = 1 7/8
Answer: 1 7/8
6. Multiply Three Mixed Numbers, Fractions, and Whole Numbers
Definition: Multiply three or more numbers (mix of mixed numbers, fractions, whole numbers) by converting all to fractions and multiplying together.
Multiply all numerators together / Multiply all denominators together
✏️ Example: 1 1/2 × 2 × 2/3
Convert: 1 1/2 = 3/2, 2 = 2/1, 2/3 stays
Multiply numerators: 3 × 2 × 2 = 12
Multiply denominators: 2 × 1 × 3 = 6
Result: 12/6 = 2
Answer: 2
7. Multiplication with Mixed Numbers: Word Problems
Definition: Apply mixed number multiplication to solve real-world problems.
📝 Steps to Solve:
- Read the problem carefully
- Identify what numbers need to be multiplied
- Convert all mixed numbers to improper fractions
- Multiply and simplify
- Write answer with appropriate units
✏️ Example: Running Problem
Maria runs 2 1/4 miles each day. How far does she run in 5 days?
Solution:
Convert: 2 1/4 = 9/4
Multiply: 9/4 × 5/1 = 45/4
Convert: 45/4 = 11 1/4
Answer: 11 1/4 miles
8. Multiply Fractions and Mixed Numbers in Recipes
Definition: Recipe problems involve scaling ingredients up or down, commonly using mixed numbers and fractions.
✏️ Example: Doubling a Recipe
A recipe calls for 1 1/2 cups of flour. If you want to make 2 1/2 times the recipe, how much flour do you need?
Solution:
Convert: 1 1/2 = 3/2, 2 1/2 = 5/2
Multiply: 3/2 × 5/2 = 15/4
Convert: 15/4 = 3 3/4
Answer: 3 3/4 cups of flour
✏️ Example: Partial Recipe
A cake recipe needs 2 2/3 cups of sugar. You want to make 3/4 of the recipe. How much sugar?
Convert: 2 2/3 = 8/3
Multiply: 8/3 × 3/4 = 24/12 = 2
Answer: 2 cups of sugar
9. Multiply Fractions and Mixed Numbers: Multi-Step Word Problems
Definition: Problems requiring multiple operations with mixed numbers and fractions to reach the solution.
📝 Problem-Solving Strategy:
- Break the problem into smaller steps
- Solve each step in order
- Use the result from one step in the next
- Check if your final answer makes sense
✏️ Example: Multi-Step Garden Problem
A garden is 3 1/2 meters long. Another garden is 1 1/3 times as long. A third garden is 2/3 the length of the second garden. How long is the third garden?
Solution:
Step 1: Find second garden length
3 1/2 × 1 1/3 = 7/2 × 4/3 = 28/6 = 14/3 = 4 2/3 meters
Step 2: Find third garden length
4 2/3 × 2/3 = 14/3 × 2/3 = 28/9 = 3 1/9 meters
Answer: 3 1/9 meters
Quick Reference Chart
Operation Type | Key Steps | Example |
---|---|---|
Estimate | Round to nearest whole, then multiply | 4 2/3 × 5 1/4 ≈ 5 × 5 = 25 |
Mixed × Whole | Convert to improper, multiply | 2 1/3 × 4 = 7/3 × 4 = 28/3 |
Mixed × Fraction | Convert mixed to improper, multiply | 3 1/2 × 2/5 = 7/2 × 2/5 = 7/5 |
Mixed × Mixed | Convert both, multiply, convert back | 2 3/4 × 1 1/2 = 11/4 × 3/2 = 33/8 |
💡 Essential Formulas:
Mixed to Improper
W a/b = (W × b + a)/b
Improper to Mixed
Divide numerator by denominator
Multiplication
(a/b) × (c/d) = (a×c)/(b×d)
Estimation
Round to nearest whole number
🔑 Key Tips for Success:
- Always convert mixed numbers to improper fractions before multiplying
- Write whole numbers as fractions (n/1) for easier multiplication
- Multiply all numerators together, then all denominators together
- Simplify fractions by dividing by the GCF (Greatest Common Factor)
- Convert improper fractions back to mixed numbers in final answers
- Use estimation to check if your answer is reasonable
- In word problems, identify key information and what operation is needed
- For recipes, multiplying scales up ingredients, dividing scales down
- In multi-step problems, solve one step at a time and use results in next steps
- Always include units (cups, miles, meters) in your final answer for word problems
📚 Fifth Grade Multiply Mixed Numbers - Complete Study Guide
Master these concepts for math excellence! ✨