The marks of 50 students of a Varanasi school are given as a "less than" table:
less than 20: 6, less than 40: 18, less than 60: 33, less than 80: 44, less than 100: 50.
Convert to ordinary classes and find the mean marks.

Differencing the cumulative entries gives the ordinary classes 0-20: 6, 20-40: 12, 40-60: 15, 60-80: 11, 80-100: 6 (total 50). Class marks 10, 30, 50, 70, 90 give $\sum f_i{x}_i=60+360+750+770+540=2480$, so mean $=\frac{2480}{50}=49.6$. "65.1" treats the cumulative entries themselves as frequencies (that gives $\sum f_i=151$, not 50); "59.6" uses the upper limits as $x_i$; "496" divides by the 5 classes.