Question

The following data gives the information on the life - time (in hours) of 75 electrical instruments.

Lifetime (in hours)	0-20	20-40	40-60	60-80	80-100	100-120
Frequency	10	15	12	21	8	9

find the mean lifetime of the instruments.

Answer 1

$Lifetime$ $\left(inhours\right)$	$Class-mark$ $x_i$	$Frequency$ $f_i$	$f_i{x}_i$
$0-20$	$10$	$10$	$100$
$20-40$	$30$	$15$	$450$
$40-60$	$50$	$12$	$600$
$60-80$	$70$	$21$	$1470$
$80-100$	$90$	$8$	$720$
$100-120$	$110$	$9$	$990$
		$\sum f_i=75$	$\sum f_i{x}_i=4330$

$Mean=\frac{\sum f_i{x}_i}{\sum f_i}=\frac{4330}{75}=57.73$