Concept: 1 Mark
Application: 3 Marks
Life timeNumber of & Cumulative(in hours) lampsFrequency1500−200014142000−250056702500−3000601303000−3500862163500−4000742904000−4500623524500−500048400
Now, n= 400
So, n2=4002=200
This observation lies in the class 3000-3500
So, 3000-3500 is the median class
Therefore, l = 3000, h = 500, cf = 130, f = 86
∴ Median=l+(n2−cff)×h=3000+(200−13086)×500
=3000+406.98=3406.98 hours
Hence, the median life time of a lamp is 3406.98 hours