What is 15/40 plus 9/21? (Answer: 45/56)

Exact Answer

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Problem: What is 15/40 plus 9/21?
Formula / Expression: 15/40 + 9/21
Final Answer: 45/56
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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 15/40 by 21, and multiply the numerator and denominator of 9/21 by 40 so both fractions share the common base 840.
Working: 15 × 21/40 × 21 = 315/840 and 9 × 40/21 × 40 = 360/840

Step 3: Add the Numerators
Now add the numerators (315 + 360 = 675) and keep the common denominator 840 unchanged.
Working: 315 + 360/840 = 675/840

Step 4: Simplify the Fraction to Lowest Terms
Find the Greatest Common Divisor (GCD) of 675 and 840, which is 15. Divide both the numerator and denominator by 15.
Working: 675 ÷ 15/840 ÷ 15 = 45/56

Step 5: Decimal & Percentage Representation
Dividing the numerator 45 by denominator 56 produces a repeating decimal (approximately 0.803571, or 80.3571%).
Working: 45 ÷ 56 = 0.803571 (80.3571%)

Key Values:
- Simplified Fraction: 45/56
- Mixed Number Form: 45/56
- Decimal Value: 0.803571
- Percentage: 80.3571%

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Solved with FractionsCalc:
https://fractionscalc.com/what-is-15-40-plus-9-21/
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15/40 + 9/21 = 45/56

Simplified Fraction

45/56

Mixed Number

Proper (45/56)

Decimal Value

0.803571

Percentage

80.3571%

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Step-by-Step Solution: 15/40 + 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 15/40 by 21, and multiply the numerator and denominator of 9/21 by 40 so both fractions share the common base 840.

15 × 21/40 × 21 = 315/840 and 9 × 40/21 × 40 = 360/840

Step 3

Add the Numerators

Now add the numerators (315 + 360 = 675) and keep the common denominator 840 unchanged.

315 + 360/840 = 675/840

Step 4

Simplify the Fraction to Lowest Terms

Find the Greatest Common Divisor (GCD) of 675 and 840, which is 15. Divide both the numerator and denominator by 15.

675 ÷ 15/840 ÷ 15 = 45/56

Step 5

Decimal & Percentage Representation

Dividing the numerator 45 by denominator 56 produces a repeating decimal (approximately 0.803571, or 80.3571%).

45 ÷ 56 = 0.803571 (80.3571%)

Visual Representation

The diagram below illustrates 15/40 plus 9/21 visually:

Summary & Alternative Representations

Property / Format	Value
Lowest Terms Fraction	`45/56`
Mixed Number Form	`45/56`
Decimal Form	`0.803571`
Percentage	`80.3571%`

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Frequently Asked Questions

What is 15/40 plus 9/21 as a fraction?

15/40 + 9/21 in simplest fraction form is 45/56.

What is 15/40 plus 9/21 as a decimal?

15/40 + 9/21 converted to a decimal is 0.803571.

What is 15/40 plus 9/21 as a percentage?

15/40 + 9/21 expressed as a percentage is 80.3571%.

Does 15/40 plus 9/21 result in a terminating or repeating decimal?

It results in a repeating decimal (approximately 0.803571...) because the denominator contains prime factors other than 2 and 5.