91 / 9/9 =

The fraction $\frac{9}{9}$ simplifies to 1.

$$\frac{28}{91} \div \frac{9}{9} = \frac{28}{91} \div 1$$

Dividing any number by 1 results in the same number.

$$\frac{28}{91} \div 1 = \frac{28}{91}$$

Find the greatest common divisor (GCD) of the numerator 28 and the denominator 91.
The factors of 28 are $1, 2, 4, 7, 14, 28$.
The factors of 91 are $1, 7, 13, 91$.
The GCD of 28 and 91 is 7.
Divide both the numerator and the denominator by 7.

$$\frac{28 \div 7}{91 \div 7} = \frac{4}{13}$$

The simplified fraction is $\frac{4}{13}$.