With $N=25$ (an odd number of patients), the median is the single $(N+1)/2 = 13$th observation, found from the cumulative frequency table. …

Since $N = \sum f = 25$ is odd, the median is the value of the $\left(\dfrac{N+1}{2}\right) = \left(\dfrac{26}{2}\right) = 13$th observation, located from the cumulative frequency column.

Step 1 — Build the cumulative frequency table.

Check: $\sum f = 3+6+9+4+3 = 25$, matching the given 25 patients. …

Using the even-$N$ averaging rule when $N$ is actually odd (here wrongly averaging two values instead of picking the single 13th observation) is the main pitfall — always determine whether $N$ …