What is difference between the number of males reading newspaper B and the number of males reading newspaper D?

Number of readers :

Newspaper B $\Rightarrow \frac{20000 \times 20}{100}=4000$

Newspaper D $\Rightarrow \frac{20000 \times 12}{100}=2400$

$\therefore$ Required difference $=\left(4000 \times \frac{2}{5}-2400 \times \frac{1}{3}\right)$

$=(1600-800)=800$