# Math Help - LCD & Adding/Subtracting Whole Numbers?

1. ## LCD & Adding/Subtracting Whole Numbers?

How do I find the LCD?? I really suck at math!! HELP!!
(One of my HW problems)

7. 5/7 - 2/3 = ? How do i use LCD on them?

2. ## LCD in fractions

LCD is least common divisor

7.5/7 - 2/3

Here the denominators are 7 and 3 the lowest common divisor of these is 7 x 3 = 21

Make both denominators 21

7.5/7 x 3/3 - 2/3 x 7/7
= 22.5/21 - 14/21
= 8.5/21

3. Originally Posted by ClulessOnMath
How do I find the LCD?? I really suck at math!! HELP!!
(One of my HW problems)

7. 5/7 - 2/3 = ? How do i use LCD on them?
Somehow, I think the 7 is the problem number. Am I right?

In any case, $\frac{5}{7}-\frac{2}{3}$ has denominators of 7 and 3.

This one is easy since they are both prime numbers. You just multiply them together to get 21 as your new denominator. Now convert each fraction in your subtraction expression to the new denominator.

$\frac{5}{7}=\frac{?}{21}$ and $\frac{2}{3}=\frac{?}{21}$

Determine what number you need to multiply the denominator by to get the new denominator. In the first one, it would be 3.

$\frac{5}{7} \cdot \frac{3}{3}=\frac{15}{21}$

In the second one, it would be 7.

$\frac{2}{3} \cdot \frac{7}{7}=\frac{14}{21}$

Now use your two converted fractions to obtain the difference.

$\frac{15}{21} - \frac{14}{21}=\frac{15-14}{21}=\boxed{\frac{1}{21}}$

4. LCD of $5/7 - 2/3$

First you wanne get the same denominator...
You will do this by timing the first fraction by 3 and the second one by 7.

$15/21 - 14/21$

Then you will just simply Subtract the Fractions like this...

$15-14/21$

$1/21$