First locate the median class and compute the median, then average the frequency-weighted absolute deviations of the class mid-points from the median. …

Step 1 — Median. Here $n = 50$, so the median is the size of the $\dfrac{n}{2} = 25\text{th}$ value. Cumulative frequencies are 5, 15, 35, 44, 50, so the 25th value lies in class 40–60. With $L = 40$, $c.f. = 15$, $f = 20$ and $i = 20$: $Median = 40 + \dfrac{25 - 15}{20} \times 20 = 40 + 10 = 50$.

Step 2 — Absolute deviations of mid-points from the median (50), weighted by frequency.