The table shows the ages of 60 visitors to a library, in classes of width 10 years…

Step 1. The median class. The median $35$ is in the class $30$ to $40$. So $l = 30$, $f = 12$, $h = 10$, $\dfrac{N}{2} = 30$ and $cf = 5 + x + 11 = 16 + x$.

Step 2. The equation. $35 = 30 + \dfrac{30 - (16 + x)}{12} \times 10$, so $5 = \dfrac{14 - x}{12} \times 10$.

Step 3. Find $x$. $60 = 140 - 10x$ gives $x = 8$.

Step 4. Find $y$. $5 + 8 + 11 + 12 + y + 7 = 60$ gives $43 + y = 60$ and $y = 17$.