What is 21/31 plus 15/21? (Answer: 302/217)

Exact Answer

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Problem: What is 21/31 plus 15/21?
Formula / Expression: 21/31 + 15/21
Final Answer: 1 85/217 (302/217)
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Step-by-Step Solution:

Step 1: Find the Least Common Denominator (LCD)
The denominators 31 and 21 are coprime (they share no common factors other than 1). Therefore, their Least Common Denominator (LCD) is simply their direct product: 31 × 21 = 651.
Working: LCD(31, 21) = 651

Step 2: Convert to Equivalent Fractions with Common Denominator 651
Multiply the numerator and denominator of 21/31 by 21, and multiply the numerator and denominator of 15/21 by 31 so both fractions share the common base 651.
Working: 21 × 21/31 × 21 = 441/651 and 15 × 31/21 × 31 = 465/651

Step 3: Add the Numerators
Now add the numerators (441 + 465 = 906) and keep the common denominator 651 unchanged.
Working: 441 + 465/651 = 906/651

Step 4: Simplify the Fraction to Lowest Terms
Find the Greatest Common Divisor (GCD) of 906 and 651, which is 3. Divide both the numerator and denominator by 3.
Working: 906 ÷ 3/651 ÷ 3 = 302/217

Step 5: Convert Improper Fraction to a Mixed Number
Since the numerator (302) is greater than the denominator (217), divide 302 by 217: it goes in 1 whole time(s) with a remainder of 85, giving the mixed number 1 85/217.
Working: 302/217 = 1 85/217

Step 6: Decimal & Percentage Representation
Dividing the numerator 302 by denominator 217 produces a repeating decimal (approximately 1.391705, or 139.1705%).
Working: 302 ÷ 217 = 1.391705 (139.1705%)

Key Values:
- Simplified Fraction: 302/217
- Mixed Number Form: 1 85/217
- Decimal Value: 1.391705
- Percentage: 139.1705%

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Solved with FractionsCalc:
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21/31 + 15/21 = 302/217

Simplified Fraction

302/217

Mixed Number

1 85/217

Decimal Value

1.391705

Percentage

139.1705%

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Step-by-Step Solution: 21/31 + 15/21

Step 1

Find the Least Common Denominator (LCD)

The denominators 31 and 21 are coprime (they share no common factors other than 1). Therefore, their Least Common Denominator (LCD) is simply their direct product: 31 × 21 = 651.

LCD(31, 21) = 651

Step 2

Convert to Equivalent Fractions with Common Denominator 651

Multiply the numerator and denominator of 21/31 by 21, and multiply the numerator and denominator of 15/21 by 31 so both fractions share the common base 651.

21 × 21/31 × 21 = 441/651 and 15 × 31/21 × 31 = 465/651

Step 3

Add the Numerators

Now add the numerators (441 + 465 = 906) and keep the common denominator 651 unchanged.

441 + 465/651 = 906/651

Step 4

Simplify the Fraction to Lowest Terms

Find the Greatest Common Divisor (GCD) of 906 and 651, which is 3. Divide both the numerator and denominator by 3.

906 ÷ 3/651 ÷ 3 = 302/217

Step 5

Convert Improper Fraction to a Mixed Number

Since the numerator (302) is greater than the denominator (217), divide 302 by 217: it goes in 1 whole time(s) with a remainder of 85, giving the mixed number 1 85/217.

302/217 = 185/217

Step 6

Decimal & Percentage Representation

Dividing the numerator 302 by denominator 217 produces a repeating decimal (approximately 1.391705, or 139.1705%).

302 ÷ 217 = 1.391705 (139.1705%)

Visual Representation

The diagram below illustrates 21/31 plus 15/21 visually:

Summary & Alternative Representations

Property / Format	Value
Lowest Terms Fraction	`302/217`
Mixed Number Form	`1 85/217`
Decimal Form	`1.391705`
Percentage	`139.1705%`

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Frequently Asked Questions

What is 21/31 plus 15/21 as a fraction?

21/31 + 15/21 in simplest fraction form is 302/217 (or 1 85/217 as a mixed number).

What is 21/31 plus 15/21 as a decimal?

21/31 + 15/21 converted to a decimal is 1.391705.

What is 21/31 plus 15/21 as a percentage?

21/31 + 15/21 expressed as a percentage is 139.1705%.

How do you write 21/31 plus 15/21 as a mixed number?

Because the numerator is greater than the denominator, the improper fraction 302/217 can be written as the mixed number 1 85/217.

Does 21/31 plus 15/21 result in a terminating or repeating decimal?

It results in a repeating decimal (approximately 1.391705...) because the denominator contains prime factors other than 2 and 5.