The $2\times2$ shortcut $\chi^2=N(ad-bc)^2/(\text{product of margins})$

Consider the $2\times2$ contingency table on two attributes A and B:

$$\begin{array}{c|cc} & A_1 & A_2\\\hline B_1 & 10 & 20\\ B_2 & 30 & 40 \end{array}$$

What is the value of $\chi^2$ for testing the independence of the attributes A and B?