The number of telephone calls received at an exchange per interval for 250 successive one-minute intervals are given in the following frequency table:

Let the assume mean be $A=3$.

We know that mean, $\bar{X}=A+\frac{1}{N} \sum_{i=1}^{n} f_{i} d_{i}$

Here, we have $N=\sum f_{i}=250, \sum f_{i} d_{i}=135$ and $A=3$.

Putting the values in the formula, we get

$\bar{X}=A+\frac{1}{N} \sum_{i=1}^{n} f_{i} d_{i}$

$=3+\frac{1}{250} \times 135$

$=3+0.54$

$=3.54$

Hence, the mean number of calls per interval is 3.54.