We have n = 400
So, n/2 = 400/2 = 200
The cumulative Frequency just greater than n/2 is 216 then the median class is 3000-3500 such that
the lower limit (l) = 3000
cumulative frequency of the class preceding 3000-3500 (cf) = 130
frequency of the median class 3000-3500 = 86,
class size (h) = 500
Using the formula,
= 3000 + 406.98
= 3406.98
So, median life time of lamps is 3406.98 hours