Here,
ClassFrequencyCumulativefrequency0−510105−10152510−15123715−20205720−25966
Now, N2=662=33, which lies in the interval 10-15. Therefore, lower limit of the median class is 10.
The highest frequency is 20, which lies in the interval 15-20. Therefore, lower limit of modal class is 15.
Hence, sum of lower limits is 10+15=25.