The lifetimes of 70 dry-cell batteries of a particular brand were tested and the following cumulative frequency data were obtained.
Lifetime up to (h)
50 : 6
100 : 18
150 : 32
200 : 44
250 : 57
300 : 65
350 : 70
(i) Plot the above data on graph paper to obtain the cumulative frequency curve (ogive).
(ii) From the graph, estimate the median lifetime of the batteries.