Here, we observe that, all 100 residents of a town have age equal and above 0. Since, 90 residents of a town have age equal and above 10.

So, 100 – 90 = 10 residents lie in the interval 0 - 10 and so on. Continue in this manner, we get frequency of all class intervals. Now, we construct the frequency distribution table.

Here, (assumed mean) a = 35

and (class width) h = 10

By step deviation method,

= 35 - 4 = 31

Hence, the required mean age is 31 yr.