What is 3/7 plus 10/21? (Answer: 19/21)

Exact Answer

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Problem: What is 3/7 plus 10/21?
Formula / Expression: 3/7 + 10/21
Final Answer: 19/21
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Step-by-Step Solution:

Step 1: Find the Least Common Denominator (LCD)
Notice that 21 is an exact multiple of 7 (7 × 3 = 21). The Least Common Denominator (LCD) is simply 21, meaning we only need to scale one fraction.
Working: LCD(7, 21) = 21

Step 2: Convert to Equivalent Fractions with Common Denominator 21
Multiply the numerator and denominator of 3/7 by 3, and multiply the numerator and denominator of 10/21 by 1 so both fractions share the common base 21.
Working: 3 × 3/7 × 3 = 9/21 and 10 × 1/21 × 1 = 10/21

Step 3: Add the Numerators
Now add the numerators (9 + 10 = 19) and keep the common denominator 21 unchanged.
Working: 9 + 10/21 = 19/21

Step 4: Check if the Fraction Can Be Simplified
The Greatest Common Divisor (GCD) of 19 and 21 is 1, confirming that 19/21 cannot be reduced further—it is already in its simplest irreducible form.
Working: 19/21

Step 5: Decimal & Percentage Representation
Dividing the numerator 19 by denominator 21 produces a repeating decimal (approximately 0.904762, or 90.4762%).
Working: 19 ÷ 21 = 0.904762 (90.4762%)

Key Values:
- Simplified Fraction: 19/21
- Mixed Number Form: 19/21
- Decimal Value: 0.904762
- Percentage: 90.4762%

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Solved with FractionsCalc:
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3/7 + 10/21 = 19/21

Simplified Fraction

19/21

Mixed Number

Proper (19/21)

Decimal Value

0.904762

Percentage

90.4762%

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Step-by-Step Solution: 3/7 + 10/21

Step 1

Find the Least Common Denominator (LCD)

Notice that 21 is an exact multiple of 7 (7 × 3 = 21). The Least Common Denominator (LCD) is simply 21, meaning we only need to scale one fraction.

LCD(7, 21) = 21

Step 2

Convert to Equivalent Fractions with Common Denominator 21

Multiply the numerator and denominator of 3/7 by 3, and multiply the numerator and denominator of 10/21 by 1 so both fractions share the common base 21.

3 × 3/7 × 3 = 9/21 and 10 × 1/21 × 1 = 10/21

Step 3

Add the Numerators

Now add the numerators (9 + 10 = 19) and keep the common denominator 21 unchanged.

9 + 10/21 = 19/21

Step 4

Check if the Fraction Can Be Simplified

The Greatest Common Divisor (GCD) of 19 and 21 is 1, confirming that 19/21 cannot be reduced further—it is already in its simplest irreducible form.

19/21

Step 5

Decimal & Percentage Representation

Dividing the numerator 19 by denominator 21 produces a repeating decimal (approximately 0.904762, or 90.4762%).

19 ÷ 21 = 0.904762 (90.4762%)

Visual Representation

The diagram below illustrates 3/7 plus 10/21 visually:

Summary & Alternative Representations

Property / Format	Value
Lowest Terms Fraction	`19/21`
Mixed Number Form	`19/21`
Decimal Form	`0.904762`
Percentage	`90.4762%`

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Frequently Asked Questions

What is 3/7 plus 10/21 as a fraction?

3/7 + 10/21 in simplest fraction form is 19/21.

What is 3/7 plus 10/21 as a decimal?

3/7 + 10/21 converted to a decimal is 0.904762.

What is 3/7 plus 10/21 as a percentage?

3/7 + 10/21 expressed as a percentage is 90.4762%.

Does 3/7 plus 10/21 result in a terminating or repeating decimal?

It results in a repeating decimal (approximately 0.904762...) because the denominator contains prime factors other than 2 and 5.