A card is chosen randomly from a pack of playing cards. The probability that it is a black king or queen of heart or jack is: 

Let $A, B, C$ are the events of choosing a black king, a queen of heart and a jack respectively. 

$\therefore P\left(A\right)=\frac{2}{52},P\left(B\right)=\frac{1}{52},P\left(C\right)=\frac{4}{52}$

These are mutually exclusive events,

$\therefore P(A\cup B\cup C)=\frac{2}{52}+\frac{1}{52}+\frac{4}{52}=\frac{7}{52}.$