Find the digits A and B, if :

$\begin{matrix} & 8 & \text{B} & 4 \ - & 5 & 1 & \text{B} \ \hline & \text{B} & 2 & 1 \ \hline \end{matrix}$

$\begin{matrix} & 8 & \text{B} & 4 \ - & 5 & 1 & \text{B} \ \hline & \text{B} & 2 & 1 \ \hline \end{matrix}$

Clearly, 4 - B is a number whose ones digit is 1.

⇒ 4 - B = 1 , 4 - B = 11 , 4 - B = 21; and so on.

⇒ B = 4 - 1 , B = 4 - 11, B = 4 - 21; and so on.

⇒ B = 3 , B = -7, B = -17; and so on.

$\begin{matrix} & 8 & \text{3} & 4 \ - & 5 & 1 & \text{3} \ \hline & \text{3} & 2 & 1 \ \hline \end{matrix}$

Since, B is a digit. ∴ B = 3