### Home > CC1 > Chapter 3 > Lesson 3.2.1 > Problem3-97

3-97.

Compute each sum or difference.

Before adding or subtracting, convert one or both fractions so that they have a common denominator.

1. $\quad \frac { 9 } { 10 } + \frac { 7 } { 9 }$

• The Lowest Common Denominator is $90$.

$\frac{151}{90}=1\frac{61}{90}$

1. $\quad \frac { 1 } { 2 } - \frac { 3 } { 11 }$

• The Lowest Common Denominator is $22$.

1. $\quad \frac { 2 } { 5 } - \frac { 1 } { 15 }$

• The Lowest Common Denominator is $15$.

$\frac{2}{5}\cdot \frac{3}{3}=\frac{6}{15}$

$\frac{6}{15}-\frac{1}{15}=\frac{5}{15}$

$\frac{5}{15}=\frac{1}{3}$