Solve the Fractional Equation: 3 1/2y = 21

Linear Equations with Mixed Number Coefficients

Solve the equation

312y=21 3\frac{1}{2}\cdot y=21

❤️ Continue Your Math Journey!

We have hundreds of course questions with personalized recommendations + Account 100% premium

Step-by-step video solution

Watch the teacher solve the problem with clear explanations
00:05 Let's solve this problem together.
00:14 First, break down three point five into a whole number and a fraction.
00:21 Next, convert it using a half number fraction.
00:30 Now, connect the fractions together.
00:43 Multiply everything by two to eliminate the fraction.
00:53 Then, reduce what's possible.
00:58 Now, isolate the unknown Y and find its value.
01:09 And that's how we find the solution!

Step-by-step written solution

Follow each step carefully to understand the complete solution
1

Understand the problem

Solve the equation

312y=21 3\frac{1}{2}\cdot y=21

2

Step-by-step solution

To solve the equation 312y=21 3\frac{1}{2} \cdot y = 21 , we'll follow these steps:

  • Convert the mixed number to an improper fraction.
  • Divide both sides of the equation by the coefficient of y y .

Let's analyze these steps in detail:

Step 1: Convert the mixed number to an improper fraction.
The coefficient of y y is 312 3\frac{1}{2} . Converting to an improper fraction, we have:

312=72 3\frac{1}{2} = \frac{7}{2}

Step 2: Divide both sides of the equation by 72 \frac{7}{2} .
The equation becomes:

72y=21 \frac{7}{2} \cdot y = 21

To isolate y y , divide both sides by 72 \frac{7}{2} :

y=21÷72 y = 21 \div \frac{7}{2}

Dividing by a fraction is equivalent to multiplying by its reciprocal, so:

y=2127 y = 21 \cdot \frac{2}{7}

Carrying out the multiplication, we calculate:

y=2127=427 y = \frac{21 \cdot 2}{7} = \frac{42}{7}

Dividing the numerator by the denominator gives us:

y=6 y = 6

Thus, the solution to the equation is y=6 y = 6 .

3

Final Answer

y=6 y=6

Key Points to Remember

Essential concepts to master this topic
  • Rule: Convert mixed numbers to improper fractions first
  • Technique: Convert 312 3\frac{1}{2} to 72 \frac{7}{2} , then multiply by reciprocal
  • Check: Substitute answer back: 312×6=21 3\frac{1}{2} \times 6 = 21

Common Mistakes

Avoid these frequent errors
  • Adding the whole number and fraction separately when converting
    Don't convert 312 3\frac{1}{2} by just doing 3 + 1/2 = 4/2! This gives you the wrong fraction. Always use the formula: improper fraction = (whole×denominator)+numeratordenominator \frac{(whole \times denominator) + numerator}{denominator} = (3×2)+12=72 \frac{(3 \times 2) + 1}{2} = \frac{7}{2} .

Practice Quiz

Test your knowledge with interactive questions

Solve for X:

\( x - 3 + 5 = 8 - 2 \)

FAQ

Everything you need to know about this question

Why can't I just work with the mixed number as is?

+

Mixed numbers are harder to work with in equations! Converting 312 3\frac{1}{2} to 72 \frac{7}{2} makes division much cleaner and prevents calculation errors.

How do I remember the formula for converting mixed numbers?

+

Think of it as "multiply, add, keep": multiply the whole number by the denominator, add the numerator, and keep the same denominator. For 312 3\frac{1}{2} : (3×2) + 1 = 7, so 72 \frac{7}{2} !

Why do we multiply by the reciprocal instead of just dividing?

+

Dividing by a fraction is the same as multiplying by its reciprocal! So 21÷72 21 ÷ \frac{7}{2} becomes 21×27 21 × \frac{2}{7} . This makes the calculation much easier to handle.

What if my answer comes out as a fraction - is that wrong?

+

Not at all! Many equations have fractional answers. Just make sure to simplify if possible and always check by substituting back into the original equation.

Can I solve this by dividing 21 by 3.5 instead?

+

Yes, you can! 312=3.5 3\frac{1}{2} = 3.5 , so 21÷3.5=6 21 ÷ 3.5 = 6 . However, working with fractions keeps your work exact and avoids any decimal rounding errors.

🌟 Unlock Your Math Potential

Get unlimited access to all 18 Linear Equations (One Variable) questions, detailed video solutions, and personalized progress tracking.

📹

Unlimited Video Solutions

Step-by-step explanations for every problem

📊

Progress Analytics

Track your mastery across all topics

🚫

Ad-Free Learning

Focus on math without distractions

No credit card required • Cancel anytime

More Questions

Click on any question to see the complete solution with step-by-step explanations