Two cards are drawn at random from a pack of 52 cards. The probability that one of them  is black and other is red is

Two cards are drawn at random from a pack of 52 cards.

$n\left(S\right)={ }^{52}{C}_2$

E=One of them  is black and other is red 

$n\left(E\right)={ }^{26}{C}_1{\times }^{26}{C}_1$

$P\left(E\right)=\frac{n\left(E\right)}{n\left(S\right)}=\frac{26\times 26}{26\times 51}=\frac{26}{51}$