median & interquartile range

Please tell me where I make mistake?
to calculate the median and interquartile range for the salaries.(this question from Q&A bank part 5 ,Question no. 5.8)
Date
annual salaries number
male female total
under 10000 6 14 20
10001-25000 25 25 50
25001-50000 15 10 25
over 50000 4 1 5

For cumulative frequency
cumulative frequency
upto 10000 20
upto 25000 70
upto 50000 95
over 50000 100
Now here n=100 (even) ,so for median we use
(n+1)/2th observation=101/2=50.5 which lies 10001-25000 range
by interpolation
Median =10000 +(50.5-20)(25000-10000)/(70-20)=19150
Similarly,for lower quartile, we use
(n+2)/4th observation=102/4=25.5(which lies 10001-25000)
lower quartile=10000+(25.5-20)(25000-10000)/(70-20)=11650
upper quartile=25000+(75.5-70)(50000-25000)/(95-70)=29500
interquartile range =29500-11650=17850
NOTE: in solution ,why we take 50th observation for median ,25th observation for lower quartile,75th observation for upper quartile.

 

Thanks for the reply Sir.
Actually Sir I am confuse in this topic.Because when n=even then in some question take 25 ,50,75th percentiles as

and in some question take 25.25,50.5,75.75th percentiles when n=even (I am sending the question in which n=even & use 25.25,50.5 &75.75th percentilies)the question isthis question belongsto ASI october 2006)
The following frequestion table gives the starting salaries(in 0000 Rs/annum)of a sample 42 graduating seniors of a liberal arts college.
starting salary 47 48 49 50 51 52 54 56 57 60
frequency 4 1 3 5 8 10 5 2 3 1

Please, clarify this point when we take 25,50 ,75th percentiles and when we take 25.25,50.5 75.75th percentiles.