What is 12/40 plus 9/21? (Answer: 51/70)

Exact Answer

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Problem: What is 12/40 plus 9/21?
Formula / Expression: 12/40 + 9/21
Final Answer: 51/70
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Step-by-Step Solution:

Step 1: Find the Least Common Denominator (LCD)
The denominators 40 and 21 are coprime (they share no common factors other than 1). Therefore, their Least Common Denominator (LCD) is simply their direct product: 40 × 21 = 840.
Working: LCD(40, 21) = 840

Step 2: Convert to Equivalent Fractions with Common Denominator 840
Multiply the numerator and denominator of 12/40 by 21, and multiply the numerator and denominator of 9/21 by 40 so both fractions share the common base 840.
Working: 12 × 21/40 × 21 = 252/840 and 9 × 40/21 × 40 = 360/840

Step 3: Add the Numerators
Now add the numerators (252 + 360 = 612) and keep the common denominator 840 unchanged.
Working: 252 + 360/840 = 612/840

Step 4: Simplify the Fraction to Lowest Terms
Find the Greatest Common Divisor (GCD) of 612 and 840, which is 12. Divide both the numerator and denominator by 12.
Working: 612 ÷ 12/840 ÷ 12 = 51/70

Step 5: Decimal & Percentage Representation
Dividing the numerator 51 by denominator 70 produces a repeating decimal (approximately 0.728571, or 72.8571%).
Working: 51 ÷ 70 = 0.728571 (72.8571%)

Key Values:
- Simplified Fraction: 51/70
- Mixed Number Form: 51/70
- Decimal Value: 0.728571
- Percentage: 72.8571%

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Solved with FractionsCalc:
https://fractionscalc.com/what-is-12-40-plus-9-21/
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12/40 + 9/21 = 51/70

Simplified Fraction

51/70

Mixed Number

Proper (51/70)

Decimal Value

0.728571

Percentage

72.8571%

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Step-by-Step Solution: 12/40 + 9/21

Step 1

Find the Least Common Denominator (LCD)

The denominators 40 and 21 are coprime (they share no common factors other than 1). Therefore, their Least Common Denominator (LCD) is simply their direct product: 40 × 21 = 840.

LCD(40, 21) = 840

Step 2

Convert to Equivalent Fractions with Common Denominator 840

Multiply the numerator and denominator of 12/40 by 21, and multiply the numerator and denominator of 9/21 by 40 so both fractions share the common base 840.

12 × 21/40 × 21 = 252/840 and 9 × 40/21 × 40 = 360/840

Step 3

Add the Numerators

Now add the numerators (252 + 360 = 612) and keep the common denominator 840 unchanged.

252 + 360/840 = 612/840

Step 4

Simplify the Fraction to Lowest Terms

Find the Greatest Common Divisor (GCD) of 612 and 840, which is 12. Divide both the numerator and denominator by 12.

612 ÷ 12/840 ÷ 12 = 51/70

Step 5

Decimal & Percentage Representation

Dividing the numerator 51 by denominator 70 produces a repeating decimal (approximately 0.728571, or 72.8571%).

51 ÷ 70 = 0.728571 (72.8571%)

Visual Representation

The diagram below illustrates 12/40 plus 9/21 visually:

Summary & Alternative Representations

Property / Format	Value
Lowest Terms Fraction	`51/70`
Mixed Number Form	`51/70`
Decimal Form	`0.728571`
Percentage	`72.8571%`

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Frequently Asked Questions

What is 12/40 plus 9/21 as a fraction?

12/40 + 9/21 in simplest fraction form is 51/70.

What is 12/40 plus 9/21 as a decimal?

12/40 + 9/21 converted to a decimal is 0.728571.

What is 12/40 plus 9/21 as a percentage?

12/40 + 9/21 expressed as a percentage is 72.8571%.

Does 12/40 plus 9/21 result in a terminating or repeating decimal?

It results in a repeating decimal (approximately 0.728571...) because the denominator contains prime factors other than 2 and 5.