What is 15/57 plus 8/21? (Answer: 257/399)

Exact Answer

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Problem: What is 15/57 plus 8/21?
Formula / Expression: 15/57 + 8/21
Final Answer: 257/399
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Step-by-Step Solution:

Step 1: Find the Least Common Denominator (LCD)
The denominators 57 and 21 share a common factor (GCD = 3). Finding their Least Common Multiple gives an LCD of 399 (which is smaller than their product 57 × 21 = 1197, keeping the calculation simpler).
Working: LCD(57, 21) = 399

Step 2: Convert to Equivalent Fractions with Common Denominator 399
Multiply the numerator and denominator of 15/57 by 7, and multiply the numerator and denominator of 8/21 by 19 so both fractions share the common base 399.
Working: 15 × 7/57 × 7 = 105/399 and 8 × 19/21 × 19 = 152/399

Step 3: Add the Numerators
Now add the numerators (105 + 152 = 257) and keep the common denominator 399 unchanged.
Working: 105 + 152/399 = 257/399

Step 4: Check if the Fraction Can Be Simplified
The Greatest Common Divisor (GCD) of 257 and 399 is 1, confirming that 257/399 cannot be reduced further—it is already in its simplest irreducible form.
Working: 257/399

Step 5: Decimal & Percentage Representation
Dividing the numerator 257 by denominator 399 produces a repeating decimal (approximately 0.64411, or 64.411%).
Working: 257 ÷ 399 = 0.64411 (64.411%)

Key Values:
- Simplified Fraction: 257/399
- Mixed Number Form: 257/399
- Decimal Value: 0.64411
- Percentage: 64.411%

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Solved with FractionsCalc:
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15/57 + 8/21 = 257/399

Simplified Fraction

257/399

Mixed Number

Proper (257/399)

Decimal Value

0.64411

Percentage

64.411%

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Step-by-Step Solution: 15/57 + 8/21

Step 1

Find the Least Common Denominator (LCD)

The denominators 57 and 21 share a common factor (GCD = 3). Finding their Least Common Multiple gives an LCD of 399 (which is smaller than their product 57 × 21 = 1197, keeping the calculation simpler).

LCD(57, 21) = 399

Step 2

Convert to Equivalent Fractions with Common Denominator 399

Multiply the numerator and denominator of 15/57 by 7, and multiply the numerator and denominator of 8/21 by 19 so both fractions share the common base 399.

15 × 7/57 × 7 = 105/399 and 8 × 19/21 × 19 = 152/399

Step 3

Add the Numerators

Now add the numerators (105 + 152 = 257) and keep the common denominator 399 unchanged.

105 + 152/399 = 257/399

Step 4

Check if the Fraction Can Be Simplified

The Greatest Common Divisor (GCD) of 257 and 399 is 1, confirming that 257/399 cannot be reduced further—it is already in its simplest irreducible form.

257/399

Step 5

Decimal & Percentage Representation

Dividing the numerator 257 by denominator 399 produces a repeating decimal (approximately 0.64411, or 64.411%).

257 ÷ 399 = 0.64411 (64.411%)

Visual Representation

The diagram below illustrates 15/57 plus 8/21 visually:

Summary & Alternative Representations

Property / Format	Value
Lowest Terms Fraction	`257/399`
Mixed Number Form	`257/399`
Decimal Form	`0.64411`
Percentage	`64.411%`

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Frequently Asked Questions

What is 15/57 plus 8/21 as a fraction?

15/57 + 8/21 in simplest fraction form is 257/399.

What is 15/57 plus 8/21 as a decimal?

15/57 + 8/21 converted to a decimal is 0.64411.

What is 15/57 plus 8/21 as a percentage?

15/57 + 8/21 expressed as a percentage is 64.411%.

Does 15/57 plus 8/21 result in a terminating or repeating decimal?

It results in a repeating decimal (approximately 0.64411...) because the denominator contains prime factors other than 2 and 5.