Ages (in completed years) of 40 members of a yoga club, written in inclusive form:
21-30: 6, 31-40: 11, 41-50: 13, 51-60: 7, 61-70: 3.
Find the mean age.

Class marks 25.5, 35.5, 45.5, 55.5, 65.5 (the same whether you use 21-30 or the boundaries 20.5-30.5). $\sum f_i{x}_i=153+390.5+591.5+388.5+196.5=1720$ and $\sum f_i=40$, so the mean is $\frac{1720}{40}=43$ years. "38.5" uses the lower limits 21, 31, 41, ... as $x_i$; "47.5" uses the upper limits; "344" divides by the 5 classes.