This is continuous/grouped data, so the median itself must first be located by interpolation inside the median class, before the mean deviation about it can be found.

Step 1 — Build the cumulative frequency table and locate the median class.

$N=50$, so $N/2=25$. The c.f. first reaches or exceeds 25 at the class 20–30 (c.f.$=28$), so this is the median class: $L=20$, $cf=13$ (c.f. of the class before), $f=15$, $h=10$.

Step 2 — Apply the interpolation formula.

Step 3 — Find $|x-M|$ and $f|x-M|$ using class marks.

| Class | $f$ | Class mark $x$ | $|x-28|$ | $f|x-28|$ |

|---|---|---|---|---|

| 0–10 | 5 | 5 | 23 | 115 |

| 10–20 | 8 | 15 | 13 | 104 |

| 20–30 | 15 | 25 | 3 | 45 | …

Using the class LIMITS of the median class (20 and 30) instead of correctly substituting into $L + \left(\frac{N/2-cf}{f}\right)\times h$ term-by-term is the most common interpolation-formula error. Also remember: once $M$ is found, the mean-deviation calculation still uses CL …