For a grouped distribution the range spans from the lower limit of the lowest class to the upper limit of the highest class, and the quartiles are found by interpolation after building the cumulative frequencies.

Step 1 — Range. Range is the difference between the upper limit of the highest class and the lower limit of the lowest class:

Step 2 — Cumulative frequencies.

Step 3 — First quartile $Q_1$. In a continuous series $Q_1$ is the size of the $\dfrac{n}{4}\text{th} = \dfrac{40}{4} = 10\text{th}$ value, which lies in the class 10–20 (c.f. reaches 13 there). With $L = 10$, $c.f. = 5$, $f = 8$ and $i = 10$:

Step 4 — Third quartile $Q_3$. $Q_3$ is the size of the $\dfrac{3n}{4}\text{th} = \dfrac{3 \times 40}{4} = 30\text{th}$ value, which lies in the class 40–60 (c.f. reaches 36 there). With $L = 40$, $c.f. = 29$, $f = 7$ and $i = 20$:

Step 5 — Quartile deviation.