A distribution uses the unequal classes 0-10, 10-20, 20-40, 40-70 with frequencies 12, 18, 26, 14. The median class works out to be 20-40. Which value of h must be used in the median formula?

Cumulative frequencies 12, 30, 56, 70 give $n=70$ and $\frac{n}{2}=35$, confirming the median class 20-40. In Median = $l+\left(\frac{n/2-cf}{f}\right)\times h,h$ is the width of the MEDIAN CLASS itself: $40-20=20$. Equal class sizes are convenient but not required. 10 is the width of the first two classes, 30 is the width of the last class, and 17.5 is the average of the four widths — none of these belongs in the formula.