The marks of 40 students are grouped in inclusive classes. Find the mode.

Step 1. The boundaries. The modal class is $31$ to $40$, with boundaries $30.5$ and $40.5$. So $l = 30.5$ and $h = 10$.

Step 2. The formula. $\text{mode} = 30.5 + \dfrac{16 - 9}{32 - 9 - 7} \times 10$.

Step 3. The number. $\dfrac{7}{16} \times 10 = 4.375$, so $\text{mode} = 30.5 + 4.375 = 34.875 = 34.88$.