Let the number of seats in the stadium = x

Number of seats in the lower deck = a and the number of seats in the upper deck be b

x=a+b

a=$$\ \frac{\ x}{4}$$ and b = $$\ \frac{\ 3x}{4}$$

Now in the lower deck $$\ \frac{\ 4a}{5}$$ seats were sold and $$\ \frac{\ a}{5}$$ seats were unsold.

Number of total seats sold in the stadium = $$\ \frac{\ 2x}{3}$$ 

Number of unsold seats in the lower deck =  $$\ \frac{\ a}{5}$$ =  $$\ \frac{\ x}{20}$$

Number of unsold seats in the stadium =  $$\ \frac{\ x}{3}$$

Required fraction = $$\ \frac{\ \ \frac{\ x}{20}}{\ \frac{\ x}{3}}$$

= $$\ \frac{\ 3}{\ \ 20}$$

A is the correct answer.