The table shows the time that 60 callers waited, in minutes…

Step 1. The heights. $\dfrac{N}{4} = 15$ and $\dfrac{3N}{4} = 45$.

Step 2. $Q_1$. The height $15$ lies in the class $20$ to $30$, where the running total goes from $14$ to $29$. So $Q_1 = 20 + \dfrac{15 - 14}{15} \times 10 = 20.67$ minutes.

Step 3. $Q_3$. The height $45$ lies in the class $40$ to $50$, where the running total goes from $43$ to $54$. So $Q_3 = 40 + \dfrac{45 - 43}{11} \times 10 = 41.82$ minutes.

Step 4. The interquartile range. $41.82 - 20.67 = 21.15$ minutes, using the unrounded values $41.8182 - 20.6667 = 21.1515$.