The cumulative frequencies with their respective class intervals are as follows.

It can be observed that the cumulative frequency just greater than n/2 (i.e.400/2 = 200) is 216, belonging to class interval 3000 – 3500.
Median class = 3000 – 3500
Lower limt (l) of median class = 3000
Frequency (f) of median class = 86
Cumulative frequency (cf) of class preceding median class = 130
Class size = 500
Median = l + ((n/2 – cf)/f) × h = 3000 + ((200 – 130)/86) × 500 = 3000 + 35000/86 = 3000 + 406.978 + 3406.978
Therefore, median lifetime of lamps is 3406.98 hours.