A shop records the time that 60 customers spend at the counter…

Step 1. The total. $5 + x + 20 + 15 + y + 5 = 60$ gives $x + y = 15$.

Step 2. The median class. The median $28.5$ lies between $20$ and $30$, so the median class is $20$ to $30$. Then $l = 20$, $f = 20$, $h = 10$, $\dfrac{N}{2} = 30$ and $C = 5 + x$.

Step 3. Use the formula. $28.5 = 20 + \dfrac{30 - (5 + x)}{20} \times 10$. So $8.5 = \dfrac{25 - x}{2}$, which gives $25 - x = 17$ and $x = 8$. Then $y = 15 - 8 = 7$.

Step 4. The mean. The frequencies are $5, 8, 20, 15, 7, 5$. Then $\sum f x = 25 + 120 + 500 + 525 + 315 + 275 = 1760$, so $\bar{x} = \dfrac{1760}{60} = 29.33$ minutes.