A histogram represents the distribution of a variable $X$ with class intervals $[0, 10), [10, 20), [20, 30), [30, 40)$. The frequencies are $f_1=20, f_2=40, f_3=30, f_4=10$. If we define the frequency density $FD = \frac{f}{w}$, what is the total area of the histogram if the width $w=10$ is constant?