What is 12/21 plus 15/21?
12/21 plus 15/21 equals 9/7 (or 1 2/7 as a mixed number) (or 1.285714 in decimal form). Learn the step-by-step solution to add 12/21 and 15/21 using Least Common Denominator and simplification.
Exact Answer
12/21 + 15/21 = 9/7
Simplified Fraction
9/7
Mixed Number
1 2/7
Decimal Value
1.285714
Percentage
128.5714%
Step-by-Step Solution: 12/21 + 15/21
Step 1
Check the Denominators (Like Fractions)
Both fractions already share the identical denominator (21). Because these are like fractions, no conversion to a common denominator is required—we directly add their numerators while keeping the denominator 21.
12/21 + 15/21
Step 2
Add the Numerators
Now add the numerators (12 + 15 = 27) and keep the common denominator 21 unchanged.
12 + 15/21 = 27/21
Step 3
Simplify the Fraction to Lowest Terms
Find the Greatest Common Divisor (GCD) of 27 and 21, which is 3. Divide both the numerator and denominator by 3.
27 ÷ 3/21 ÷ 3 = 9/7
Step 4
Convert Improper Fraction to a Mixed Number
Since the numerator (9) is greater than the denominator (7), divide 9 by 7: it goes in 1 whole time(s) with a remainder of 2, giving the mixed number 1 2/7.
9/7 = 12/7
Step 5
Decimal & Percentage Representation
Dividing the numerator 9 by denominator 7 produces a repeating decimal (approximately 1.285714, or 128.5714%).
9 ÷ 7 = 1.285714 (128.5714%)
Visual Representation
The diagram below illustrates 12/21 plus 15/21 visually:
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Summary & Alternative Representations
| Property / Format | Value |
|---|---|
| Lowest Terms Fraction | 9/7 |
| Mixed Number Form | 1 2/7 |
| Decimal Form | 1.285714 |
| Percentage | 128.5714% |
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Frequently Asked Questions
What is 12/21 plus 15/21 as a fraction?
12/21 + 15/21 in simplest fraction form is 9/7 (or 1 2/7 as a mixed number).
What is 12/21 plus 15/21 as a decimal?
12/21 + 15/21 converted to a decimal is 1.285714.
What is 12/21 plus 15/21 as a percentage?
12/21 + 15/21 expressed as a percentage is 128.5714%.
How do you write 12/21 plus 15/21 as a mixed number?
Because the numerator is greater than the denominator, the improper fraction 9/7 can be written as the mixed number 1 2/7.
Does 12/21 plus 15/21 result in a terminating or repeating decimal?
It results in a repeating decimal (approximately 1.285714...) because the denominator contains prime factors other than 2 and 5.